Here are some possible ways to fix the “SIM not supported” issue on your iPhone 13 Pro Max:
Check for carrier settings: Open Settings > General > About > Carrier Lock. If your iPhone is locked, contact your wireless carrier about unlocking your iPhone. If you don’t see this option, or if it says something else, try the following methods.
Eject and reinsert the SIM card: Look for the SIM card tray on the side of your iPhone. Use a SIM card ejector tool or a straightened out paperclip to open the tray. Push the tray back in to reseat the SIM card.
Force restart your iPhone: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the Apple logo.
Reset network settings: Open Settings > General > Transfer Or Reset iPhone > Reset > Reset Network Settings. Enter your iPhone passcode, then tap Reset Network Settings to confirm your decision.