Synology RT6600AX DNS

Good morning
I have a Synology Router and a Synology nas
both are on a separate ddns.
If I want to go to the router with ddns it works fine.
if I want to use the ddns of my Nas internally, this works well because I have a DNS server running on the router.
but if I want to externally via DDNS to my nas then that does not work.

This worked but not anymore since 2 days.
If I do nslookup it arrives at my router properly. (ip is from provider on my router)

So this setup can be pretty complicated once you add in a DNS server. How are you trying to connect to the NAS and what DNS configs do you have on the local