New 735, can't sync activities to Garmin connect on mobile, but steps do?

Assuming you're starting your activity, then stopping and saving it?

Yes the activities are saved on watch but not syncing?

Have you tried connecting your 735xt to your computer, then syncing to Garmin Connect web using Garmin Express? Using the same account used for GC mobile.

Yes that works but I hoped to do it on the phone, thanks

Yes, understood. At least your activities are getting saved to the cloud. You might want to try restarting your 735XT then remove it from GC mobile, then add it back.