If the SIM card is not recognized in the device after activation, you can try the following steps:
1. under "Network & Internet", click on "Mobile Network" and activate the "Use SIM" button.
2. checks that the SIM card is correctly inserted into the slot.
3. If the SIM is not recognized automatically, check that the APNs are correct.
The best is to try the SIM card in another device. If the error comes from the device, contact the dealer or manufacturer.
If nothing works: a SIM card defect is rare, but not excluded. Contact the support to get a replacement card.