Help with S4 not connecting to AT&T network

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Hey,
I'm trying to fix my friend's S4 and I double checked everything, but it refuses to connect to the (US) AT&T network. By refuses, I mean the signal drops very often and is hard to connect, although sometimes it manages to do so.

It's an unlocked phone with the correct radios for AT&T.

Model: GT-I9508
APN: phone
MMSC: https://mmsc.mobile.att.net
MMS Proxy: proxy.mobile.att.net
MMS Port: 80
MCC: 310
MNC: 410
APN Type: default,supl,mms,hipri

Am I doing something incorrect here or could there be a faulty antenna or sim card?

[DenialS.]

Maybe the phone just sucks..

[Assistive]

Most likely a bad SIM. Is it a ghetto rig Micro SIM (cut down) or a real one. I would see about trying a different SIM card as that can fix the problem.

Another thing to take into consideration is the location. Service is not always great in some areas and can also affect if you get signal or not.

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Most likely a bad SIM. Is it a ghetto rig Micro SIM (cut down) or a real one. I would see about trying a different SIM card as that can fix the problem.

Another thing to take into consideration is the location. Service is not always great in some areas and can also affect if you get signal or not.

It's ghettoly cut down. Service-wise is fine. I have another AT&T iPhone 5 running full bars with LTE (can't swap since Apple decided to use a freaking nano SIM). I'll try another SIM card and see how that works.

[Assistive]

It's ghettoly cut down. Service-wise is fine. I have another AT&T iPhone 5 running full bars with LTE (can't swap since Apple decided to use a freaking nano SIM). I'll try another SIM card and see how that works.

Ghetto SIMs are never reliable. Just have your friend go to an AT&T store and ask for a Micro SIM for a GS4. All he needs is the account information and they will get it to him free of charge.