I have a Plex Server on my BE800 LAN, which I access from outside using IPv6. My IPv6 settings are

WAN : PPPoE/Auto
LAN : SLAAC+Stateless DHCP

Now, the problem is, I need to explicitly allow inbound rules to let my mobile devices access the Plex Server using its IPv6. The other part to the problem is, the IPv6 for the Plex server seems to be changing (I am not sure why it changes, rebooting the NAS and the router did not change it) over a few days. Consequently, the rule becomes invalid and needs to be updated.

 Questions :

1. Short of static IPv6 assignment on my NAS, is there a way to ensure that the NAS's IPv6 never changes?
2. Is there a way to use wildcards in the firewall rule to allow inbound connections for the entire subnet, thus making Plex work even if the server IP changes?