IMac can't detect any Wi-Fi networks?

I've been having a problem with my 2011 iMac where my Wi-Fi will gradually stop working. I know that this is not a router problem, as my MacBook, iPhone and other devices all connect perfectly and my iMac worked with this router for years up until now. When I start the computer, it is able to connect to Wi-Fi, but after about 20 mins, my main 5G connection stops working, so then I choose my other connection, which works for about another 20 mins. Gradually, all of the various connections slowly disappear until my connection drops out and my computer won't detect any networks in my vacinity at all. So far, the only thing that works is restarting my computer. I've been though this process many times now and the Wi-Fi will always follow that exact pattern until it stops working. Network diagnostics doesn't do anything since I can't get any kind of connection to begin with. Unfortunately I don't have easy access to Ethernet where my computer is in my house unless I go buy a really long cable, so I would prefer if I can possibly fix the Wi-Fi.

I have a 2011 27 inch iMac with High Sierra installed, the problem began when I had Sierra and I thought an update may fix it but it still persists.

Try looking up the exact model number of your iMac, and ok up and download a new network driver (before doing that uninstall the old one, under settings) and then restart and install the new one. After it is installed restart the machine again, and try reconnecting.

"IMac can't detect any Wi-Fi networks… All of the various connections slowly disappear"

How many is "all the various"? Does your router broadcast more than two frequencies? No matter how firmly you believe the network is perfect, you should do a *HARD* reset of the router. Then follow the user guide for that router to choose a password for the Wi-Fi network.

If you think I'm nuts, try following the suggestion of the other answer instead.