logging into Internet gateway

Assuming you have the Arcadyan gateway or possibly the newer Sagemcom one, the only way to access the admin options is from the T-Mobile Home Internet app. For whatever reason, T-mobile removed access via the web page awhile ago.

The configuration options for those gateways is very limited, but it was just as limited via the web page. In the app select the "NETWORK" tab. The options you find there are all that you get - Network name, password, frequency band, encryption. You can also create a new network (SSID) by clicking the plus sign in the corner.

If you want greater control, you'll have to use a standalone router, but that comes with some issues and doesn't solve all problems. Search the forum for discussions relating to that.