Alternative DNS fallback when NXDOMAIN?

[Michel-0]

Hi there,

this is more like a general question, not necessarily related to AdGuard, but i hope to reach some competent people around here.

Thing is i'm running AdGuard home on my raspberry pi, which is connected to my router.
So i configured its (local) IP as DNS server in my router settings.
Works perfectly for all connected devices to my router (blocks just fine).
However i want my network to stay online (beeing able to resolve DNS requests) even if my raspberry is offline.
So i configured a public DNS server as alternative DNS server in my router.

This seems to be a problem, no ads get blocked anymore now.
Is it possible / a common thing, that a router tries to resolve a DNS using the second DNS server if the primary DNS server returns NXDOMAIN?
If i configure the AGH as primary DNS on my PC and a public DNS as secondary, ads get properly blocked, so it might not be a general thing / rule to try the second if first returns NXDOMAIN?

It's a AVM FRITZ!Box 7272 (v06.88).
It might changed with the firmware update i've done recently on my router... because i think it worked just fine couple of months ago.

Anyone knows anything about that?
May i contact my router manufacturer?

Kind regards,
Michael-0