Having the same issue with an iPad Pro, but my phone is fine. I can connect to an older Apple router, but not my recent Bell router. I get failures, and if I "forget this network" and try again, I get password errors. Everything is fine on my 25 or so other WiFi devices.
I decided to troubleshoot some more, and checked the logs on the router for a clue. I got what looks like a clue. I get numerous errors saying a device got the password wrong. Sometimes several a second. Sometimes it says a device connected successfully, then in the same second another line saying the password was wrong. For clarity, these are not me typing a password - it has to be the device trying to use its memorized password (that usually works).
This began with the 15.3 update.
Perhaps this may help others in their troubleshooting. Maybe it's that the new Bell router supports a newer WiFi technology that the iPad is trying to use, and that's why the older Apple router still connects? Maybe there's a bug in 15.3 that affects the passing of credentials with the newer technology? Just guessing of course, but there has to be a reason.