You can use the provided DB dump CronJob template:
oc process -f https://raw.githubusercontent.com/tobru/nextcloud-openshift/master/mariadb-backup.yaml | oc -n MYNAMESPACE create -f -
This script dumps the DB to the same PV as the database stores it's data. You must make sure that you copy these files away to a real backup location.
To backup files, a simple solution would be to run f.e. restic in a Pod
as a CronJob and mount the PVCs as volumes. Then use an S3 endpoint for restic
to backup data to.