What is usually meant by "disabling double NAT", would be using the ISP device as modem only - and then terminating the PPPoE session on your OpenWrt router. This isn't really possible using recent'ish Fritz!OS versions anymore (well, there is an undocumented/ unsupported trick, but, well…) - and doing so would also make all the non-modem features of your Fritz!Box unusable (phone pbx, ATA, DECT, etc.).
In this situation using an OpenWrt as downstream router, with static routes configured on the Fritz!Box and without masquerading (double NAT) on the OpenWrt router is the next best alternative. Not great (as the proprietary devices needs to be 'good enough' to do handle the PPPoE session and NAT), but the only way to keep the additional (mostly phone-) features working.
.. I gave this a try after some time again and the problem was me of course.
I set a static IPv4 to the WAN interface of the OpenWRT device and forgot to set the default gateway.