I'd like to offer some clarification on UW.
Let's start with Verizon. Verizon classifies both n77 and mmWave bands n260 and n261 as UW. The Spitz is not capable of n260 and n261 but IS capable of n77. n77 combined with other non-mmWave bands can supports speeds in excess of 1000 Mbps. So, you will see plenty fast speeds on Verizon when n77 is present (which is becoming more and more common).
Now T-Mobile. T-Mobile also uses mmWave but I've never encountered it. That said, the majority of the T-Mobile towers I've encountered have n41 (often called mid-band). As with Verizon's n77, T-Mobile's n41 can exceed 1000 Mbps as well so you will see plenty of speed.
In summary, both Verizon's n77 and T-Mobile's n41 will provide very good speeds with the X3000 Spitz or Puli.
Edit: Forgot to add that US-Mobile does work as I'm currently using a US Mobile sim in my X3000 on their Lightspeed (T-Mobile) network. I have never used a Verizon sim but have used a Visible (Verizon 'MVNO') in the past.