Thanks for taking the time to post to our community! We are always upgrading our network to keep our customers on the latest and greatest frequencies! I am not very familiar with the area unfortunately but if you could get me some ZIP codes, I can take a closer look at the coverage we have there and see if there any plans for upgrades.
First, the entire area you reference is covered by T-Mobile's 700A license, so any lack of service in the area is due either to a lack of priority on T-Mobile's part or a lack of cooperation on the part of local zoning boards.
The area is also within the coverage of a number of OTA TV stations out of Seattle, that may need to be relocated (re-channeled) before 600 MHz can be deployed. The area inside the purple arcs is generally impacted by this. These stations are not scheduled tor relocation until Jan '20. So, it will probably be several months before any 600 MHz (band 71) will be seen in the area.
*I have not looked at each station in detail to see exactly how they affect T-Mobile's licenses.