Nextcloud on a CGNAT ISP (OMV5 docker in a RPI 4)

My ISP Provider is under CGNAT. So cant forward ports.
But I’ve got ipv6.
Is there any way to get Nextcloud work? I have a DuckDNS account.
Completely newbie. Tried everything I found in the net, no success.
Any, any help, will be appreciated.
Sorry for my baaaaad english.