Overriding ISP DNS

[DualIP]

Over a year ago I freetzed my FB7340, using xs4all with native IPv6

Problem I run into:
DHCP6 advertises fritzbox as DNS server. Fritzbox uses ISP DNS, so internal domain names (like MS AD) aren't accessible.

I already tried to make FB use internal DNS for its lookups in 3 ways:
-Freetz webgui-AVM services-settings: Multid DNS setting, pointed to internal DNS
-Edited ar7.cfg: In part "servercfg" confired DNS server settings to point to internal DNS
-Added line to onlinechange daemon: echo 192.168.1.11 > /var/tmp/avm-resolv.conf

However, FB keeps using external DNS server, causing DNS lookup not functioning for internal host names

Any hints welcome....