Can I use an unlocked T-Mobile Galaxy S5 on AT&T? I made the unfortunate mistake of buying a Galaxy S5 through AT&amp;T and was rewarded with a locked bootloader and no fun whatsoever for any reason ever. 

As a remedy, I've considered purchasing a T-Mobile Galaxy S5 and somehow unlocking  it (how does that process work again?) for use on AT&amp;T. 

Two questions:


Will it be compatible with most (if not all) bands on AT&amp;T? (ie: will I lack anything or will I be able to use the network just as well as with the AT&amp;T S5?)
How do I unlock a device for use on a different carrier?


According to AT&amp;T, these are the supported bands for the AT&amp;T S5:


4G-LTE: 1, 2, 3, 4, 5, 7, and 17
4G: HSPA+ with enhanced backhaul
3G - UTMS: 840, 1900, 2100MHz
GSM/GPRS/EDGE: 850, 900, 1800, 1900MHz


According to T-Mobile, these are the supported bands for the T-Mobile S5:


LTE: 1, 2, 3, 4, 5, 7, 8, and 17
UMTS: Band 1 (2100), Band 2 (1900), Band 4 (1700/2100), Band 5 (850)
"Quad Band GSM" (what bands?)


It seems to be compatible.

See also AT&amp;T Radio Frequencies and T-Mobile Radio Frequencies

Will it work?
Okay, here's what I can gather...


There should be no real loss of capability if you move to AT&amp;T. While AT&amp;T does not advertise LTE Band 8 capability on its S5s, Band 8 is largely used in South Korea, Japan, and Taiwan; unless you travel to those places, it shouldn't be a problem. I would, however, check AT&amp;T's coverage map to be certain.
T-Mobile allows customers to unlock phones provided certain conditions are met. Click here for more information.


Also, "Quad Band GSM" on T-Mobile is 850/900/1800/1900 MHz, same as AT&amp;T