SIM Not Supported

Hello to All, I've just bought a refurbished iPhone 8 Plus online and when I put a T Mobile prepaid SIM in there I get the "SIM Not Supported" message.  In addition, my iPhone is "stuck" on the SIM Not Supported page.  I tried without success to go "back", double click home button and power on/off.  Any suggestions ?  Thanks in advance.

You have provided the answer already :

 

bought a refurbished iPhone 8 Plus online

The seller seemed to have a pretty good reputation-it was Decluttr.  And the phone arrived in mint condition with a new battery.  Unfortunately their customer service is terrible.  Again, any way to unfreeze the "SIM not Supported" page ?  Any way to tell if it's locked to a certain carrier ?

I don't know which network it is Locked to but, obviously not, T-Mobile.

Get back to the Seller and talk about your woes

Decluttr, the outfit I bought the phone from, finally replied after trying 5 different ways to contact them.  It's locked to ATT.  They say I should call ATT to get it unlocked.  Will this work or just another time sink ?

You'll need someone from your side of the pond, to answer that

Well, to answer my own questions...

1.  To unfreeze an iPhone 8 Plus:  Quickly press and release the volume up button. Quickly press and release the volume down button, then press and hold the side button.

2.  To see if an iPhone is locked:  Settings, General, About, Scroll Down to Carrier Lock section.

3.  To unlock an iPhone from ATT:  https://www.att.com/deviceunlock/

I thought Step #3 was going to take days but ATT unlocked it within seconds of my online request.

1. Go to the AT&T website and sign in to your account.
2. Click on the "My Devices" tab.
3. Select the iPhone 8 Plus from the list of devices.
4. Click on the "Unlock" button.
5. Follow the instructions on the screen to request an unlock code.

AT&T will usually send you the unlock code within 24 hours. Once you have the unlock code, you can insert a SIM card from another carrier into your iPhone and it will work.

I hope it works for you.

If this is the case, you will need to contact AT&T to request an unlock code. The unlock code will allow you to use the iPhone with a different carrier's SIM card.
 
Here are the steps on how to request an unlock code from AT&T:

• Go to the AT&T website and sign in to your account.
• Click on the "My Devices" tab.
• Select the iPhone 8 Plus from the list of devices.
• Click on the "Unlock" button.
• Follow the instructions on the screen to request an unlock code.

AT&T will usually send you the unlock code within 24 hours. Once you have the unlock code, you can insert a SIM card from another carrier into your iPhone and it will work.
 
I hope it works for you.