Hi. few days bak i faced the same issue.
Seems like nokia care ppl had forgotten to fix one of the screws.
and this screw was from the antenna. So remove the back cover and check if all 6 screws are in place and tightly secured.
If yes then there must me some loose connection or dust between the points on the antenna and the point on the main board.
Also check if all the SIM connection pins in the sim card slot are at the same level.

HTH