Macbook Air will not connect to Wifi even though other Macs work fine with same Wifi I'm having some issues and I feel as though I have literally tried everything. I am at a complete loss on what to do. 

Situation:

I had multiple Mac computers including this one for a while. All working fine with router.

Then I replaced my router because it was getting slow and dated, the new one I got worked fine at first, but then started to drop connection every other day and even sometimes every day. I replaced the replacement router, then my macbook air had ridiculously slow connection speed. It would take google 5-10 minutes to load. But my iMac computer worked fine. I didn't change anything, nor do anything different to either computer except get this new router. The router even managed to corrupt my time machine. So I called up the ISP and they worked me through the issues, so now everything is working fine EXCEPT my Macbook Air.

I have tried starting from scratch with the router, with the Time Machine, I have rebooted all devices. But the internet remains the same. It's good on my iMac, my phone, my TV, but not the macbook air. I wanted to call Apple, but it costs $60 to talk to a someone which is not affordable for me. I am really frustrated with this. 

I went through all the changes with my Airport Utility. I even reset my Macbook Air to factory default settings. Still I have the same issue.

What can I do to troubleshoot this problem?
Have you tried going to the Network preference pane in System Preferences, clicking on Advanced, then Wi-Fi, and deleting all the Preferred Networks? I seem to recall having the same problem a few years ago, and deleting the networks fixed it.