With the new iOS 14.7.1 update being released to the public, some iPhone user has reported poor cellular signal issue and no cellular coverage on their iPhone. Such condition cannot be remediated through remote or reinstalling the SIM. This issue currently observed in iPhone 11, iPhone 8, 7 and 6S.
Apple has yet to respond to such issue. However if you have not upgraded, it might be good to upgrade only when the next version is released.
