I had a Blu Studio 5.5" that I am using on T-Mobile. I love my phone, but my coverage pales in comparison to my friend's phone (he has full bars everywhere in my house and I have 2 bars). I also had terrible coverage while traveling abroad recently. Specs here: https://www.gsmarena.com/blu_studio_5_5-5761.php
I'd like to stay on Android 5.1 (I know it's dated, but that's what I'd like). Can anyone suggest a phone that is either 5.5" or 5" that will work better with T-Mobile's bands? Or, can you point me to the latest bands so I can compare phones?
You need bands 2 4 12 66 71 for LTE support. A phone that does those and more will help when travelling and roaming. You won't find any phone older than 6.0 available to buy from TMobile. You'd have to find a lollipop phone from way back from a reseller or find a phone that TMobile branded and that came with lollipop and downgrade to the version you want. Those phones won't support the new bands 66 and 71.