This is really odd and I have gone through several methods of rectifying this issue but to no avail.
Initially, I requested a new SIM card, which did not resolve the issue.
Then I went to the Genius bar at Apple (it is an Apple 13) who confirmed there were no hardware of software issues. The software was also up-to-date.
I reset Network Settings and it still did not resolve the issue. I am only able to temporarily resolve the issue (for an hour or so) by switching the device off and back on. Then it reverts back to the issue.
Can anyone please offer help on this issue?