I recently had that same issue. I tried different things, even resetting my network settings, but nothing worked. I didn’t restore to factory settings, but I found out that if you have a VPN configuration on your device that you were using before upgrading to iOS 14, this could be what’s causing a conflict that won’t let you connect to a WiFi network. To solve this, all I did was delete the VPN configurations I had (if you have several VPN configurations as I did, delete them all, and don’t worry, you will be able to install the again to continue using your VPN settings), by going to Settings>General>VPN. There select the info icon on every configuration and then “Delete VPN”.
After I did this, I restored the network settings again, and now I can connect to my WiFi network without any problem. Also I reinstalled the VPN configurations I used and they are working just fine.
I hope this works for you as well.