It's only contiguous with the cellular A block not the B block.. TMO does have the A block B5 themselves. I think Dish will buy it as it will help with coverage even tho B71 will be more useful. I also think I heard the equipment that dish has ordered support N26 Sent from my SM-G975U1 using Tapatalk
Any tower with n71 also has B71 LTE, though the reverse isn't true.
B71 LTE is the longest range tech T-Mobile has, so from a coverage perspective a 4G phone with B71 won't be missing anything. The catch is that in many places the B71 channel is 5 MHz wide, and TMo's B71 deployment is way thinner than Sprint's B26 deployment, so having only 5 MHz hurts more. Though not as bad as having only 5 MHz of PCS.
Now, there are two more factors at play here. First, outside urban areas where Dish etc. grabbed spectrum, T-Mobile has more.to work with, so.performsnce on B71 will be better (unless some rural carrier scooped band 71 up, and some did). Additionally, T-Mobile is touting range improvements for standalone 5G, which launches this quarter, so there is a chance they'll start adding NR-SA n71 cell sites in some rural locations, similar to how they added B71 in some areas they hadn't touched before. In that case your phone wouldn't work.