Wifi camera will not connect it saying only works with 2.4GHZ

If you have the 4G LTE gateway and not one of the newer 5G gateways you can try using a browser to manage the gateway. The Nokia 5G gateway still allows the use of the browser to manage the gateway. I believe the prior 4G LTE does as well. You can access the manual for the gateway via the T-Mobile site and get more details. If you want to use the mobile application you will see the icon as below. The other T-Mobile application is for account management information etc...

If the gateway you have supports using the browser for management you might find that helpful as it provides more real estate for seeing the information. To manage the gateway you just have to login as admin in either use method.