Hi, I'm a (complete) newb to all this and very unskilled with cellphones generally, but one has to start somewhere. I have and use a Moto E second gen, aka Moto E LTE. It's an ATT version and though I have unlocked it to use on my own MNVO, it still has a bunch of ATT proprietary software on it. I was wondering whether it would be possible to somehow replace the operating system with the generic one that the original, unbranded Motorola version of the phone would have had on it. It is possible here that I'm using the term "operating system" inappropriately. I don't want to get rid of Android; it's the ATT stuff I want to get rid of.
However, that said, there is one change to the OS, I would like to be able to make. I have read online that when I upgraded from Kitkat to Lollipop, apparently the support for LTE band 12 was removed because the phone is not VOLTE-capable. Now I use an MVNO that uses T-Mobile's services, so it would be useful if I could get band 12 back. I realise that all of this will (for me, as a newb) be a long process, and I am expecting to learn a lot. If someone could start me off by telling me whether it is even possible to somehow de-ATT the phone, I would be grateful. If it is, then maybe someone could a) give me a rough idea of the process and b) point me at where I go to start learning how to do it.