If it works fine on other networks then it's your router provided by the ISP that's the problem. If you can try setting 2.4 and 5Ghz networks to separate SSIDs with different passwords. I've got a couple of devices on my network that refuse to work if the networks are combined. You could try a different WiFi access point, an older unifi can be picked up cheap on eBay on if it's just for testing. I was having similar issues with a Nintendo switch whilst working away, the hotels WiFi was just messed up

Sounds like it's an issue with your router configuration and NOT your steam deck. If you have tethering on your cell phone you can use it temporarily to test if you get Wi-Fi that way, though I wouldn't download anything while tethered to your phone if you have a data cap for your cell phone.

Since it sounds like your ISP has complete control of your router I would call their customer support and let them know you have a device that cannot connect. Most likely they'll either do something like a factory reset on the router or ask for specific hardware information (like the MAC address) about the device you're having trouble with (in this case your steam deck) so they can check the setting and ensure it isn't blacklisted or something.

If they for some reason still won't help or can't get it fixed, you can always go out and buy your own router and hook it up. You'll still have to contact your ISP again to get it setup but at least you'll both have full control over the router configuration, and you won't have to pay a monthly fee to rent a router from them.

EDIT: You may also be able to factory reset the router yourself. They usually have a pin hole in the back or something you can press with a paperclip or needle that will factory reset them when held down for a few seconds. Keep in mind anyone else connected to the router will have to redo all connections for all of their devices. If you live with other people it would probably be a good idea to let them know you want to try this before doing it so they aren't thrown off the internet unexpectedly.

EDIT2: The reason you may have had issues launching games and factory resetting the device may be due to the fact that it had a Wi-Fi connection setup but it couldn't get internet. Usually when the deck is online (even if you are playing an offline game) it will still try to contact steam servers to do things like notify friends you're playing x game and update cloud saves, as well as ensure you aren't playing games on more than one device (this last one is annoying). As a result, the game wouldn't launch because it got stuck in limbo trying to reach the steam server, thinking it could because it saw it had internet access, but never getting a response because it didn't actually have internet access. Going into the settings and setting the console to offline mode would fix this issue, assuming this was the cause. The factory reset could have been something similar, it may have been trying to check online for any update during the reset. Unfortunately both of these are conjecture and not something I can say for certain, but it at least means there's a chance nothing is wrong with your steam deck. I would try getting it connected to the internet again and testing things before reaching out to Valve for support on the steam deck, since it sounds like it is mostly an issue with your router that you are unable to connect to the Wi-Fi currently.

If your Deck connects to everything but your home connection, then yeah, it's very likely your router being the issue here. Assuming you did not have any config changes or firmware updates there then it may be dying, but I'd at least check the settings for some clues, maybe change the wifi frequency if it is fixed.

Try contacting support. Especially if it's still under warranty.

Is this an OLED deck? The OLED deck wireless chip can connect to wifi 6 networks, but when it first came out there were a lot of issues with the Deck wifi chip stopping working at all when you would connect to wifi 6 networks with specific wifi 6 features turned on. After trying to connect to wifi 6 network, the wifi would stop working altogether until a reboot.

Valve released some updates for the OLED deck that fixed it for me, but the issue you're describing sounds a lot like this. It's possible your router updated with some new firmware and enabled a new wifi feature that's crashing the deck's wifi chip. It's also worth mentioning that SteamOS recovery image doesn't have the later wifi fixes that valve released, so if you try to use a recovery usb and connect to a wifi 6 network you'll have issues.

Some network changes you can try to do (if you're able to with your ISP router) are separating your wifi into 2.4 and 5ghz networks, and after a deck restart try connecting to the 2.4ghz network (make sure the deck isn't set to automatically connect to the 5ghz channel). If that works, you can go through router settings and look for wifi 6 (AX) features and try turning them off. If you can't do anything with the router, you'll either need to get your ISP to issue you a new router (in case that one's defective) or buy your own router and use it instead (this is usually the better option anyways, most ISPs charge you a rental fee on routers that's a way worse deal than buying your own).

You can also do something like buy a wifi extender, the cheap ones are $10-15 and 2.4Ghz only. Make sure it has a different name from the main network (something like network_ext) and it should let you connect your deck.

Are the issues only with your home router / accesspoint? If the need is high you could always get a seperate accesspoint and hope that works.

Even borkier would be buying a USB-C network adapter or the dock for troubleshooting but that would get you online.

You could try flashing the recovery image and re-imaging your deck. https://help.steampowered.com/en/faqs/view/1B71-EDF2-EB6D-2BB3

I'd contact valve support if that doesn't work, they've always been great whenever I've interacted with them.

You are barking up the wrong tree.

You are saying that your Steam Deck is not working with the wifi at your home but there is no issue with it working with airport, in-flight, and hotel wifi.

That immediately tells me that there is no issue with your Steam Deck but the POS router that your ISP has provided.

You need to:

A: call your ISP and get them to fix their broken POS router.

B: At a later time get your own wifi router then call your ISP to put their useless shit into bridge mode so you can use your own router.

Have you contacted valve support?

You could try to install a different OS that's made for the deck. Bazzite for example also has a steam deck version.

Try turning off Bluetooth. Try turning off ipv6 in desktop mode. Could also be interference from electronics near your router or Deck, like a computer or your neighbour's wifi.

You need to get your ISP to help troubleshoot the issue with your router. If the Steam Deck works fine on other networks that's a very strong sign that it's not the Deck that's the issue.

Most public libraries have WiFi or computers that you can use in a pinch, leverage those as much as possible. You are paying for those services via taxes, they are yours.

Dude, the issue is not the Deck, it's your router.

2 things to note:

When offline, switch into steam offline mode to launch games, gets round some of the hanging on launch issues

Try connecting to WiFi from the steam decks desktop mode? Probably a router issue though.