AVM FRITZ!Box Tools: call deflections in "slave-mode"

Hi all,
I want to use the switches provided by “AVM FRITZ!Box Tools” to switch on/off my call deflections.
As far as I can see this feature isn’t available if the FritzBox is running in “slave-mode”.
Is there a reason for this? I can’t see any?

Some background info:
I’m using my FritzBox solely as a phone server behind my actual firewall/router. I guess that’s why the slave-mode is active.

I got the slave-mode info from the protocol: mesh_role=<MeshRoles.SLAVE: 'slave'>

By having a brief loo into the code the following condition seems to be responsible for not setting up the call deflection switches:
if statement

OP reported that: The FritzBox can be set to slave mode even when it is behind another router