Hi,
Even nextcloud is exactly the type of application where docker add problems

... as in any case, when it's so easy to use directly nextcloud ... on N.A.S. (but it's not the subject and anyone do what he want on its own NAS)
You can ask Asustor and wait, or use workaround during time you wait for a response .
ex.
1 for customer not consumer : change docker to use macvlan and create a separate I.P. address (DHCP or fixed for a server) ... so port are now not shared by A.D.M. and applications in container.
... kernel module is provide by Asustor, need just to validate promiscuous mode and configure container network (simple except if you use docker without the minimum knowledge about it ... as a consumer)
2 for basic Linux admin : set Apache web server in N.A.S. to use port 80 and 443 (default) (so myhttpd server will be off)
... configure redirection (rewrite mode) as any proxy to localhost:port attributed to nextcloud (eventually to force moving to HTTPS only) ... this is documented in nextcloud
ANOTHER possibility is : stop using port 80 ... it's not secure EVEN connected in a docker container ... certificate are easy to get to use now only HTTPS ... then use a private port for nextcloud ... so only user able to know it can use it ... no reason to keep default except opening your NAS to any attack / malware / ransomware or data violation ...
Philippe.