It has only been a month since I was last in Boston but this past weekend I went back again and things have marginally improved since September. Thanks to iOS 15 it's now more obvious when I'm connected to n41. I was spotting it in a few more areas than last time. For example, in the South End I never connected to a strong signal of n41 but I did see my phone bouncing between 1-2 bars of n41 and full signal on n71. Speeds on 1 bar of n41 were around 40Mbps in my testing. T-Mobile coverage map is starting to be a bit more accurate I guess.
Separately, most of Boston College's Main Campus is covered by a site on top of 2000 Commonwealth Ave. A month ago I was getting speeds of 40-50Mbps on campus but sometime in the past month T-Mobile fully upgraded the site. Yesterday during the commencement celebration for the class of 2020 I was getting nearly 700Mbps outdoors despite thousands of guests being on campus in addition to the thousands of on campus students and faculty. Even LTE was great at 190Mbps. Inside Conte Forum where the event was held, my phone dropped down to n71 and speeds remained consistently high at around 80-100Mbps.
5G and LTE speed test:
T-Mobile didn't bother painting the antennas post-upgrade:
There's still one other stealth site in Newton that covers part of Boston College's Upper Campus that hasn't been upgraded yet. It's still LTE only and has typical 30-40Mbps performance. I haven't seen any permits submitted for that site though so I'm not certain when it'll get upgraded.