[Android Central Question]

I have an Galaxy s6 Edge+ (G928A) from US and i use it in Romania on Orange.The problem is that the phone is working only on Edge(2G) speed dial/internet,and only because i made it to work with *#2263# settings.When it stay on normal like after an factory reset it shows 4G at signal but it doesen't work at all.The calls can be made some times and works for some seconds,other times it sais "Emergency calls only",with the internet is the same story i can use it only if it's on 2G mode.
The phone is unlocked(i think)beucase the sim works and i don't get the "insert the unlock code" when i turn on the phone.I have an code gathered from at&t request but i cant use it so i'm stuck.
Beside all the problems above i can't upgrade the android because from what i searched on internet i need to be on at&t signal with theyr sim,and that's impossible for me,and right now I'm stuck on 6.0.1 Marshmellow.Thx for your time and any help will be really apreciated.