Question [Q] SM-9006v - Using on t-mobile

Hi,

I just purchased a Galaxy s5 sm-g9006v. This is the chinese unicon Samsung galaxy s5. It's unlocked.

I have a t-mobile sim. The phone works fine but the only problem is that it's only running 2g speeds! and i've been reading and i think it's a problem with the bands the phone is working on.

I've also been reading about changing the firm-ware modem but I need a little help.

My question :
Can I change the bands that my phone is using to t-mobile compatible bands to run hspa+ or lte or 4g ?
If so, is it as simple as flashing flashing t-mobile firmware from sammobile (sm-g900t) to my phone via odin?
And if I can fix it without having to flash the whole original firmware by just flashing the modem (which is 5mb), where can i get the modem for that firmware and how do I flash it alone ?

Thank you