You want OpenWRT. They're not too limited, but they're not very powerful either. Fan controller? Probably. Pihole? You can probably hack that together, though I've never tried. Media server? Erm... not my first choice. Other stuff? Limited only by your imagination, time constraints and willingness to troubleshoot weird problems most people have never had before.

Router like this are very low spec for example Fritz!Box 3270 has 360 MHz 1 core CPU, 64 Mb of RAM and 16 Mb of Flash. Only thing you can do with them is flashing OpenWrt, but its latest version doesn't support devices with or less than 8 Mb of flash and 64 Mb of RAM.

While you can probably use them for some small tasks (like a switch, a file server with a USB thumb drive or a print server), you're probably better off if you can sell them and buy a raspberry pi or similar.

The Fritzbox 7530 for example could be sold for 50€ (at least in my country).

The problem is that they're not really made for this task, both in hardware and available software. They typically specialize in routing and switching, but have insufficient internal hardware (memory especially) to run a full-blown OS.

So whatever you install on these devices, will probably not give you all the features that you would like to have. (For example, a full linux command line with all the typical programs installed.) Also, it doesn't allow you to use HDMI to connect to a monitor, so there's that. But basic linux things can be done on it, if you figure out how to get to the command line. But it's very limited.

Start by looking here https://openwrt.org/toh/

See if any of them support ddwrt openwrt tomato router or just Linux in general. You probably couldn’t run plex media server, but you can do ad blocking, vpn server, and basic nas. Works great for these tasks.

I tried to run some software on my router. It kind of works, if it fits. Storage was the limiting factor. There's an option to expand the FS to include a USB stick, but somehow it made something overheat, and the router froze every now and then.