Thanks all I have solved it!
You won’t believe this but: when I opened my phone it shows 5g and a strong signal but when I opened google maps it tried to connect to the Honda system and there were errors and things happening etc showing on the infotainment screen and not able to cancel etc. Basically not connecting.
Anyway I looked at phone and even though there was a good signal the map was not functioning…didn’t show a route or even much else..nor did anything else work on the phone..VERY STRANGE..
Anyway today checked phone and still an issue.Strong signal but no connection.
So,I thought phone needs a repair(iPhone 12). But it worked on Wi Fi so it cannot be broken….i was pulling what’s left of my hair out.
Done know why but I checked for phone updates and lo and behold it was asking for update to iOS 18.2.
I updated phone and suddenly I got mobile data coming through again!!
So so chuffed for a technophobe like me to get it working!
All is now fine with phone and car connection..
I have emailed Apple about this ( I realise this is a Jazz forum but may help someone

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Two issues here:
1/ I have auto updates on iPhone and this update didn’t happen automatically..
2/ Apple need to be aware of this issue and email people to inform them that the mobile data can be affected as it requires to be updated to this new iOS.
And breathe!
So so relieved. It wasn’t my fault after all. Nor Honda issue.
Someone earlier said something not right and he/she was spot on!