for what its worth, I too would look at changing the battery if it's just a hotspot and occasional or spare phone
but I'm sure you have good reasons... I wouldn't likely if it was meant to also be my high use phone
We have a t-mobile plan....I don't really have a specific phone suggestion, but I'll point out something I've observed...
some phones have better antenna than others. I used to have an older (2nd gen I think) motox and my son had an iphone, can't remember which model it was... but his phone would routinely have a usable data signal when mine didn't.
Now, he has a newer iphone 6S and I have a motox pure. Just the opposite...I have signal when he doesn't, but I still don't feel like mine is as good as some...