I don't really know much about M.2 slots and the different capabilities and how the key type corresponds, so I don't want to spend money on something that won't work.
There's an unused slot on the motherboard marked 'M.2 PCIe / SATA1'. I believe it's M-keyed.
Can it be used with an M+B keyed adapter like this one? Or will it physically fit, but the socket itself isn't wired to work with an ethernet adapter? Is there a command I can run to figure out the capabilities of that slot?
My plan is to use this machine as an OpenWrt router to replace the shitty Linksys one my ISP supplied, but it would obviously need a second ethernet socket. My home wifi is dealt with by a TP-Link mesh network, so I don't need it to have wifi and bluetooth. I could use the slot that the networking card is using, but if I can retain those then that would be ideal for any future flexibility I may need.
If the m2 to Ethernet road leads nowhere: you could try one or more usb3 to Ethernet adapters.
I would recommend this over the M.2 adapter. USB3 goes up to 5 gbps, so gigabit Ethernet should be no problem. Just get an an adapter from any actual brand, even if it's just tp-link. No junk non-brand ones because they'll be unreliable or not work at all due to a bad chipset.