Can't find Web UI login info

I just got the home internet gateway and have been using the app, but found there is also a much more versatile web interface at http://192.168.12.1/ in my browser. I found I have to log in to manage the 2.4 and 5GHz networks, but I didn’t know which credentials to use.

I tried the default admin/admin, but thought that may not work, as I was already made to change the default admin password in the app when I set up the device SSID. I did not, however know what it wanted for "username" and tried admin, but that didn't work. Tried my email from my T-mobile ID, but that didn't work. Tried the phone number assigned to my gateway device, but that was rejected. Now I get the message: "web ui locked" due to all the failed login attempts.

Can anyone tell me what it is looking for in terms of username/password, so I can finally log into it?