LOVE MY PHONE SURE WISH I COULD UPDATE IT THE YEAR AND A HALF!!!!

Let me start by saying I really like my Samsung Galaxy S5 so I don't need anyone giving me crap about the phone I have please. If you must know, i can't afford a new one. Upgrade? Sure if I could. Since it's an AT&T phone and I'm with another carrier I can't get an upgrade because my phone is still on At&t and new services(lol I've been with them almost 2 years now) system can't even recognise what kind of phone I have! So i received permission from AT&T to have my phone unlocked to move to another carrier I can use the phone alright, it's just stuck on 5.0 G900ACUA PERMANENTLY because I can't get updates from either company or SAMSUNG! Any suggestions, besides buying a new phone, would be mighty welcome.

Thanks