Here's the solution that may useful to fix the Nokia C2-03 if it does not detect any sim card inserted to it and it only shows "Insert SIM card" even if the sim card is already inserted.
This solution guide shows the connection of both two sim card slot of the Nokia C2-03 since it is built as a dual SIM swapping device. Thank's to GsmAngel for sharing this solution to us.

This solution covers for the side slot SIM card.

And this one covers for the SIM card beneath the battery.