Phone connection sometimes dissapears
So the connection sometimes works, and sometimes it shows "No service" (in the same place) whats the problem? Everything is ok with wifi it does not dissapear.
In order of opperations, id look into the following.
Check for Continuity: Does you service only drop in one place? in basements only? Service has difficulty pennetrating concrete and metal. basements, and steel buildings commonly cause service issues.
Call SP: your service provider may be having intermittant issues in your area.
Check SIM: Your SIM card may be failing. Make sure to have your SP change your SIM every year.
Verify Fringe Service Zones: More country like areas have whats known as Fringe Service, which means there is low signal there on the regular.
Factory Reset: Your device software may have become corrupt. A factory reset can fix this issue.
Change Cellular Antenna: Replace the Charge flex w/ Cell antenna. this may be failing.
anything further than this is a board level failure.