Goofed a bit when setting up the new iPhone SE, an iCloud Photos setting to use mobile data was turned on, this was a carryover from the backup of the old phone. It didn’t matter on that one it already had all the photos on board.
But this one was set to use full res pics cos it has enough space. I realised what I’d done when 4.5 GB of mobile data had gone in a few hours.
No problem, I still have 9.5GB left with 6 days until the next data top up. I have an average of 500 MB per day per month & has used a whole GB in the first 24 days.