Smile [Q] Changing the ROM/Radio Does it change the bands?

First time android poster, Long time reader! (I've not changed the rom since the days of the HTC TyTN II and HTC Universal)

I have an Asian HTC One MAX. I live in the UK and use the giffgaff network which uses 800Mhz 4G LTE o2 signal. The Asian device can pick up 900Mhz 4G LTE.

I was wondering if it was possible to flash the phone with an EU Rom, which should then pick up the 800Mhz band as I guess the hardware is all the same?

- Is this possible? If so would I flash in the normal way?
- Are there any pitfalls in doing this?
-Anything I should be aware off? Or should I just give up on trying to pick the 800Mhz band up and go EE who use other bands?

Thanks for your time