Brand new bike, (my mates). Rear mech stopped working on first ride.
Any one else had probs? Can’t get it back to shop till next week.
Battery fine, front mech fine.

Have you tried turning it off,then turning it on again? 😉
Connections,usually at the shifter.You need the little tool though or you WILL pull the wire out.

I thought it needed a reboot but can’t find any info on tinternet.

Another 50p in the meter?

ctrl,alt,delete?

Have you read the instructions?
it may be in “rear derailleur setupmode” where the mech moves teeny amounts when you press the shifter, to do the same as winding your barrel adjuster on a cable mech.
hold the button on the battery indicator for 3 seconds or so, and try again.

Maybe?

Has been known, some pro conti mates had two go on a long wet early season race in Belgium.

Thanks, luckily bought from a shop with people in it. Back it goes…