Tmobile 5G Internet Gateway doesn't connect with anything old? Terrible customer service?

One option for more information, if you are still inclined, is to view Nater Tater's YouTube videos about accessing the "hidden" settings of the gateway. The config can be exported, modified, and then sent back into the gateway but this is a manual effort that takes a little technical ability. It is not super difficult and he makes the tasks much simpler but it takes care and time and in some cases a little experimentation to get results. 

Older printers may only support WPA or possibly WPA2 authentication so the 2.4GHz channel needs to also have those for the authentication. Some older printers just don't play well with the 802.11ax implementation used with the T-Mobile gateways. Wireless extenders that have wifi compatibility with the older wireless adapters can be used as a workaround for older printer connections. 

Some subscribers have used added mesh solutions that do play nice with their older equipment but this leads to more cost associated with the solution and more work to get everything established. Sometimes it is like opening Pandora's box. The more older devices you have the more challenge it becomes.