Softpedia's Marius Nestor reported suffering this problem, and added that the usual solution of enabling and then disabling Airplane Mode did not work.
Many iPhone users are reporting this same issue, which Nestor said was only solved after he rebooted his phone twice.
Nestor's second reboot of his iPhone was done hours after the first, however, which means that the resolution of the issue may have been caused by something else.
To avoid this issue, Nestor recommends that iPhone users avoid upgrading to iOS version 12.1.3 until the issue is fixed.
