Downgrade to WPA2

The T-Mobile mobile home internet application is used to set up the gateway.

The Application Avatar is like this. The one with the T only is for account info.

The actual “Home” page for the application is as below: 

There is a “Lets get you connected overlay for login or “setup” prior to this screen.

To manage the gateway you have to login as admin. When your are logged in you can see the actual home page with the options at the bottom. The Network option allows you to proceed to make the changes to the network bands/frequencies. I don't have a screen clip of that but it shows the 2.4 GHz and the two 5 GHz bands. Selecting the 2.4 GHz would allow the options to change the authentication type. After the change it is necessary to "apply or save" the change.

Not too many steps but if you have issues with the application it can be difficult to get this done. These are images from T-Mobile pages and clearly done with an Android phone as the camera at the top is clearly not an Apple notch.