I just tried to connect my iPhone to WiFi several times, and it indicated that the password was incorrect? :h
Instead of the smartphone app, I used my PC and typed in its IP address into my browser to get to the T-Mobile router admin page. You need to be connected to the router via Ethernet or WiFi.
If you don't know the IP address, you can use WIN10 search to find and run Powershell. Type ipconfig /all and locate the default gateway IP for your PC - i.e. 192.168.xx.x Type the IP into your browser address bar and press enter.
I checked my WiFi password on the admin page, and it was correct? When I typed the password again in my iPhone, it connected with no problem. Not sure why having the router admin page open made any difference, but not longer having a connection issue.
BTW, accessing the admin page through browser gives more info than the smartphone app. It indicated I've used 102G of data so far this month. It's also one of the cleanest interfaces I've seen for any router admin page, and doesn't overburden the user with too many settings.