How can I tell what band my 5g gateway is using?

Cellular metrics can be viewed in the T-mobile internet app. Select more at the bottom of the screen in that app. Then advanced cellular metrics.  It gives you both LTE metrics and also 5G metrics. The band your gateway is currently connected to will be displayed.

If you're using the Arcadyn model, you can enter this URL: http://192.168.12.1/TMI/v1/gateway?get=all

Once you're logged in to your wifi and it show display more information.

Hola,

  The url  http://192.168.12.1/TMI/v1/gateway?get=all displays the data on my screen. Very good info.

  Does anyone know a way to save this page to a file on my computer? (Windows 10).

thx