According to MacRumors:
"Modem firmware updates are a routine part of major .1 iOS releases, so it's not yet clear if the new firmware specifically addresses issues that customers have been experiencing with LTE connectivity.
Early reports on the MacRumors forums have, however, suggested that iOS 12.1 does indeed bring some improvements. MacRumors archer75, for example, says the update has doubled his LTE speeds. Other users, though, have said the update does not address LTE problems.
Regardless of whether a fix has been bundled into the iOS 12.1 update, Apple is taking reports of LTE problems seriously and investigating what might be going on, so should this be a software-related issue, a resolution is likely in the works."
The references are to iOS 12.1 beta, although the "other users'" comments may be in reference to 12.0.1.
