IPv6 forward freshtomato?

Question

Computer networks

Good afternoon, rebza!
the thing is, I have FreshTomato 2020.3 Onlime firmware on board Linksys
, it issues ipv6, it constantly changes after the restart. network supposedly in test mode.
I set up freedns.afraid.org/ for my service in LAN.
essence of the question.
if I set the port through the dns name from the AAAA record, then everything is OK until the router is rebooted or forced to click on save in the corresponding settings form.
in Ip6tables the name is converted to ipv6 of the current value.

it is possible to add a router script, so I would like to know how to overwrite and pick up a new ipv6 from an AAAA record.