Is it possible to turn off both Wi-fi bands on the new 5G router

You are 100% correct. The hidden SSID is NOT disabled. The one major point that is relevant is that with the Sagemcon gateway, especially, you have NO admin access to the gateway without some wireless connectivity. If you could turn the wireless radios off as soon as it was done you could not save the changes. You would be disconnected and the next time the gateway rebooted it would be right back where it was. This is one of the main pain points about the extremely limited management access to the gateways via the mobile application. 

T-Mobile does not really want users to have any real control over the gateway. The controls are very limited so as to almost make it no control other than changing the SSID or possibly separation of the 2.4 GHz radio from the 5GHz radios. Even if you want to add a mesh solution or another router it is pretty much impossible to do a clean job of it due to the limitations for management of the gateway.

I was told to go into the app, select network, select ssid you are trying to turn off, then enable HIDDEN (just above save), then save...do for each ssid you want to turn off (or hide)

it's embarrassing to me that they suggest that.  it won't work.  obviously, even if you hide an SSID, it is still broadcasting and working its NAT magic  they seem to think that if you hide the SSID and no devices connect to it, it won't be a problem, which is true, BUT . . . you need to have SOME SSiD even if  you are connecting via the LAN port.  Makes no sense  

But I appreciate your sharing it anyway  Very thoughtful

Ed