I install adguard on OMV 4 but how to must works for OMV5.
ADGUARD is a good alternative for pihole
links:
https://hub.docker.com/r/adguard/adguardhome
step 1: create a MacVLAN or use one previosly created
step 2: install adguard docker
step 3: go to adguad config page: http://youradguardIP:3000
step 4: use it ( configure your DHCP server to use youradguardIP as your DNS server.
done. thats all folks.