What could be causing my iMac to not find any wireless networks?

I’ve been having a problem with my 2011 iMac where my WiFi 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 WiFi, 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 WiFi 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 WiFi.

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.

Thank you for your time!