Yes that with SLAAC is known.
In fact, I want to first transmit the IPv6 of the router so that I can establish a VPN tunnel via domain.
For internal services I then use ULA addresses and an internal DNS server.
I managed to craft an ugly script.
Unfortunately it just puts data on memory, which will be deleted at the end, I would have preferred something completely in RAM.
But at least it works … 
:log info GO
/tool fetch url="https://checkipv4.dedyn.io/" mode=https src-path="/" dst-path="/deSEC-IPv4.txt"
:delay 1
:local PubIpv4 [/file get deSEC-IPv4.txt contents]
/tool fetch url="https://checkipv6.dedyn.io/" mode=https src-path="/" dst-path="/deSEC-IPv6.txt"
:delay 1
:local PubIpv6 [/file get deSEC-IPv6.txt contents]
:delay 1
/tool fetch url="https://update.dedyn.io/update?username=MYDOMAIN.dedyn.io&password=PASSOWRD&myipv4=$PubIpv4&myipv6=$PubIpv6" mode=https src-path="/" dst-path="/deSEC-Update.txt"
:delay 10
/file remove deSEC-IPv4.txt
/file remove deSEC-IPv6.txt
/file remove deSEC-Update.txt
:log info DONE