Hi,
There’s no indication about where this problem occurs (home only, local area only or when in certain areas).
If the phone has had water ingress then you may be correct, the antenna may be starting to corrode.
First thing I would check is the SIM card for oxidation/wear, quick test would be try another known good SIM, but cleaning may resolve issue or replacing worn SIM
If no change then check for iOS updates, if already up to date then try resetting network connection in settings or restarting the phone.
If no change then look at opening up phone and visual inspection for water ingress around antenna components, or take to repair shop for assessment.
Hope this helps