SIM card reader not working
I bought a used phone when I went to the ATT store to up grade my phone the SIM card reader was not working
iPhone 7
Are you sure that's a card reader issue?
Have you already tried these basic steps and see what happens?
Put your Sim card on a different iPhone
Put a different Sim card on your iPhone
Contact the carrier and ask if everything is ok with your plan, and/or if they have a software update to be installed on your device
If all these attempts fail, than have your device inspected at your most convenient Apple Store or Authorized Service Provider : Apple - Find Locations - Location Selector
Regards
Giulio