If anyone needs it , upgrading to VR600(EU)V2_200701 Beta firmware solved problem.

Thank you for your reply @David-TP I'm gonna use it for redundancy for my lan. I manually disabled first dns server from network but second dns server still inactive from clients. systemd-resolve...

Hello, DHCP server optional secondary dns option is ignored by the router. Router only sends primary dns address to its dhcp clients. This needs fixing.