SIM card not detecting after ios 18 update

Do you use any VPN, switch it off temporarily and then try.

If that doesn’t help,

Try the below steps

1. Goto Settings
2. Click on Mobile Service
3. Click on Network Selection
4. Toggle Off Automatic

Wait for few seconds, your network service provider(s) should show up in the available list.

Manually select the Network service provider and wait for the signal to appear.

Turn on Automatic again and restart your phone.

Second Option :

1. Goto Settings
2. Goto General
3. Select Transfer or Reset iPhone
4. Select Reset
5. Select Reset Network Settings
6. Restart your iPhone

If all of the above doesn’t help, call your network service provider customer care and they should be able to help you