Why is it my 5G Internet Gateway can't work using 5G?

@jbj I guess they have it right, here on their own interwebs, so depending on where you are and what you can access ... that’s what you get!  B66 is 4G/LTE @TMobile.

Don’t know if anyone has reached out to your issues - I get exactly nada from Customer Support - but they do take my money!

...

I did not know that Band 71 @600mHz could be either 5G or LTE,  if or when you can connect to it.

https://www.t-mobile.com/support/coverage/t-mobile-network

5G

• Frequencies that can provide 5G: 
  • Band n71 (600 MHz)
  • Band n41 (2.5 GHz)
  • Band n260 (39 GHz)
  • Band n261 (28 GHz)
• With 5G, high amounts of data can be transmitted more efficiently than 4G LTE. 
• One of the ways T-Mobile is rapidly deploying 5G is integrating mid-band 2.5 GHz spectrum from Sprint.
• Check out What is 5G? to learn how it works!

Extended Range 4G LTE

• Frequencies that can provide Extended Range LTE
  • Band 12 (700 MHz)
  • Band 71 (600 MHz)
• Our Extended Range LTE signal reaches 2X as far and penetrates walls for 4X better coverage in-buildings than ever before.

4G LTE

• Frequencies that can provide LTE:
  • Band 2 (1900 MHz)
  • Band 5 (850 MHz)
  • Band 4 (1700/2100 MHz)
  • Band 66 (Extension of band 4 on 1700/2100 MHz).
• 4G LTE offers fast download speeds, up to 50% faster speeds than 3G. See Data speeds.
• Voice and data services only work at the same time when on you have VoLTE enabled on your device. Otherwise, LTE only provides data.