A bit late to the party, and I don’t own a 10 ii, but wanted to add my two cents. You’re both saying that your 10 ii’s worked on T-Mobile with Android, right? When I first started skimming this thread, I was afraid it was the band 12 issue. You know about that? T-Mobile uses band 12 in non-traditional coverage areas, so they don’t want phones on their network that support band 12 and not VoLTE because if you need to make a 911 call from a band 12-only area your phone will literally have no other band to fall back to. And by law carriers have to support 911 (actually E911). I posted some threads on this back during the summer: Beta VoLTE on the Xperia 10 III, disabled by default - #85 by tao101 .
But if Sailfish connects noticeably worse than Android, I draw a blank.
From what you’re describing above about buying a throwaway, approved T-Mobile phone, it sounds like you’re tricking their back end into giving you a permanent get-out-of-jail card. You never officially told them about the 10 ii, right? So as far as T-Mobile is concerned, the throwaway phone IS your daily driver, and it seems any phone using the associated SIM gets a pass. Or did you actually buy an extra line? I think that would be even funnier, as it suggests that every phone on your account now gets a pass. I think this makes some sense from my own experiences, since for a while I was getting emails and snail mail letters about T-Mobile dropping their 3G network and please contact them for a free 5G phone. I haven’t used a 3G-only phone for quite a few years. It was a cute Nokia E73…that was T-Mobile branded. T-Mobile has the ability to see that what is connecting via my SIM card is 4G LTE, but their back ends obviously don’t talk to each other.
Did any of that help?