I've had this problem since I last updated to iOS 14.3 as well. I went ahead and "forgot" the network I was trying to login to (Settings -> Wi-Fi -> (your network) -> (Info) -> Forget This Network) and then rebooted the phone. Once rebooted, I added the network back and all is well now. I believe the official Apple answer is to do a "Reset Network Settings" but I don't want to lose all of those Wi-Fi passwords I have saved although something tells me I will have to anyways. Hope this helps.