IOS 11 update help please

[Severus111]

So today as soon as iOS11 came out, I tried to restore my dads iphone and my iphone, both 6S plus on ios 11. Both were currently running ios 10.3.3. My iphone went through smoothly.
Dads iphone is showing error "SIM not supported" with the same sim he was using before. His phone has been unlocked since last year.
I downgraded his iphone to ios 10.3.3. It still shows SIM not supported. I put in an AT&T sim and it turned on. But as soon as I put dads sim (local carrier) in it, it shows the SIM not supported error.
Please help

 

[Just_Me_D]

Put your dad’s iPhone in DFU mode, and then allow iTunes to detect it in recovery and install a fresh full copy of iOS 11 onto it.

 

[TwitchyPuppy]

He should visit his carrier store to have his SIM card replaced!

 

[Severus111]

I already did that. Tried restoring on iOS 10.3.3 and iOS 11.

 

[Severus111]

He tried some other local sims. They werent working on his phone. His sim works on my phone. So I doubt its a carrier issue.