DNS settings: OpenWrt.lan can't find xxx: Non-existent domain

[MrWrt]

Hi

I have gitlab running in docker on my synology.
I would like to access gitlab by name rather than [IP]:port.

I have setup dns server and reverse proxy on synology.

Router settings:
[OpenWrt 18.06.4 r7808-ef686b7292 / LuCI openwrt-18.06 branch (git-19.170.32094-4d6d8bc)]

Network -> Interfaces -> LAN -> DHCP Server -> Advanced settings -> DHCP-Options:
6,192.168.1.2,84.200.69.80,84.200.70.40

(Where 192.168.1.2 would be the IP of synology.)

Network -> Interfaces -> WAN -> Advanced settings -> Use custom DNS servers
192.168.1.2
84.200.69.80
84.200.70.40

Im only interested in accessing gitlab from local env. (As a start at least)
So maybe WAN settings is unnecessary(?)

This setup seems to be going up & down. From one windows machine I could access gitlab.temp.lan from chrome but not FF or powershell/Invoke-RestMethod.
One day later chrome doesnt work.

If I run ipconfig / all on that machine I get an ip like fd53:xxxx:xxxx::x
listed before the correct dns servers as stated above.

Nslookup:

nslookup gitlab.temp.lan
Server: OpenWrt.lan
Address: fd53:xxxx:xxxx::x

*** OpenWrt.lan can't find gitlab.temp.lan: Non-existent domain

It looks like my router cannot hit my synology dns server.
Googling suggests this problem is IPv6 related(?)

This could of course be related to wrong setup in synology, but I have to start somewhere.
Hope you can help.

[MrWrt]

If the answer to the question is too obvious -> Go find answers yourself

Solution:
Get IPv6 address from synology
Update here:
Network -> Interfaces -> LAN -> DHCP Server -> IPv6 Settings -> "Announced DNS servers"