PHONE LOCKED BY NETWORK CARRIER SALES PROVIDER.

Hi there I got an iPhone 11 from someone who upgraded their bill phone but I can't access the phone when I put my sim card in because the iPhone is with a different network carrier and is locked by the sales provider.

I tried resetting the iPhone multiple times (I also got instructions from the network carrier explaining how to unlock the phone) but it keeps saying "Sim card not supported". Can anyone please help me or tell me what to do.