SIM card failure

Make sure there is nothing in the sim slot, like dust, and different materials. While the SIM card is out, take a flashlight and check, then also look for a pink dot in side sim slot. This will indicate moisture issue. Use a can of compressed dry air to try and remove material. The canned air is not a solution for pick dot / moisture issue.

Careful when using things to clean SIM card. I use a good clean pencil eraser and lightly brush both sides of SIM card hold contacts ONLY before returning to slot.

If this doesn’t work, do you have another phone that you can swap sims to see if the problem follows the sim or not ?

Either way a visit to the Genius Bar at Apple may resolve this matter real quick.

If possible visit your carrier.

Ask to check if the SIM tray is bent, as even a tiny misalignment can cause the SIM card to not fully connect with the internal contacts in your iPhone.