Mobile App not allowing to add new network

I just did it yesterday for my Nokia GW, but did not use the app. I instead logged in to 192.168.12.1. I got a Wyze robot vacuum that doesn't work with 5G.

If you have the Arcadyan or Sagemcon gateway T-Mobile does not allow admin access via the web GUI interface with a browser. The T-Mobile home internet mobile application on your cell phone is how those are intended to be managed via the admin account. If you are referring to the mobile application and it is not functioning as expected try uninstalling it and then re-installing the mobile application. The most recent revision of that is 2.10 as I recall. 

When you select the Network option the screen should report the current SSID. There is the plus button at the bottom right of the screen. If the magenta circle with the white plus sign is not there see if you need to scroll down. I have never seen it require scrolling down on my iPhone. With the plus symbol you should be able to make a network and parameters.