4G not being picked up

[CasperBrixton]

Good Moring All,

A real 1st world problem here.  I've had my 2017 A6 Allroad for 3 months.  When I got the car I put a three mobile data sim in the dashboard.  All worked well for about a month with 4g enabling google earth maps to be displayed nicely.  Then the car stopped picking up 4g, leaving just 3g available, so no more google earth mapping displayed.  I've spoken with three mobile  a few times, and they've changed the sim card, but still no 4g.  Has anyone else encountered this problem?  Might it be something to do with the car, rather than the network?  Driving the same routes when 4g was initially working, so I know the network coverage is available.

Thoughts gratefully received!