Continue Disconnections Voice & Data With Kena It (only 3g)

Dec 13, 2013
suddenly my 4-year-old MI3 WCDMA showed a serious problem; even when stationary, the voice and data line is not stable, every 4-5 minutes it disconnects to reconnect after 30-40 seconds.
I tried to upgrade to the latest build (8.2.1), deleted the caches but did not solve anything.
Moving the SIM in a Nokia C5 works without problems.
As Recovery I have the TWRP; can I make a few attempts or the phone is to be thrown away?
Excuse me for the English language, I am Italian.
PS:This SIM has been installed in MI3 since November 2017 and has always worked without problems.