Systemd-nspawn and loading kernel modules?

I have an old OpenSuse 13.1 Bottle and compiled linux-dahdi kernel modules on Nixos Kernel 5.4.104.
After unpacking the kernel modules but keeping xz ending the conaitner can modprobe the modules.
However it cannot access /dev/dahdi/* (dahdi_test Operation not permitted). Eg cat /dev/d/ahdi/ranscode → operation not permitted. However the host Nixos can access it.
The systemd-devel mailinglist suggested its a conatiner issue. chmod 777 didn’t help. Doesn’t look like selinux/appormar or such would be used (didn’t find the supporting executables).
I pass -capabilities=all to nspawn. Anything else coming up your mind I could try ?

mount -t devtmpfs none /dev is how I mounted it.

Does anything come to your mind what might be preventing the container to access the /dev/dahdi/* nodes?

Using lxc instead worked.