Do you have a mesh network or access points? I

A mesh network 

Model numbers?

Sorry for delay in answering. 
play 1 ( 4 items)

connect (1 item)

ray (1 item)

1 boost

Thanks but we need to know about your mesh network devices. Many mesh networks introduce different subsets to avoid interference between the mesh device zones but this can stop interoperability between devices in different zones.

We have AVM Devices which work on two different networks. The Sonos only works on one of both networks.

Is there a model number associated with the AVM devices?

SONOS requires that all of the players and controllers are on the same subnet.

How can I verify that ?

Hook your Sonos up to two different subnets and try to get them to work, They won’t.

Hook all Sonos up to one subnet and try to get them to work, They will.

I assume that you are running an S2 system (gold icon). Go to Settings—> System —> About My System and look at the IP addresses of the controller and players. When the system is working they will be of the form xxx.yyy.zzz.123. ‘123’ will be different for each controller and player. They must be unique. In the area where the system does not work I suspect that the controller will show a different subnet xxx.yyy.aaa.123. SONOS requires everything to be on the same subnet.