So for now you have a Cloud instance installed with PBXpress on OVH. So if FQDN was previously pointing on another Public IP, then you just need to wait DNS propagation, time to wait from 5 minutes to 6H , depends on which type of 3CX license you have, ENT ttl 5 minutes, STD & PRO 6 hours.
you just neef to wait, do a nslookup time to time to see if DNS is updated.