Nokia 6021 - No operator found

Dear all,
I have been using the Nokia 6021 (V 03.83) for a couple of years
without any sort of problem.
Some time ago I started realizing that the network connection was no
more reliable. At the beginning I thought it was a problem of the
operator. I realized that it does not work with any sim card (I tried
three different operators).
If I look manually for networks from "settings"->"phone settings" -
>"manual" the phone does not find any network -> "No operator found".
The sim cards connect to the network (in the same location at the same
time) using a Nokia 6310 and a Nokia 6131.
Did you ever faced the same problem? Do you have an idea about this
problem?
I think that the radio module for the gsm does not work, but the
bluetooth radio module works perfectly like the rest of functions of
the phone.
Thank you
Fortunato