Configuration of SOPHOS UTM to work properly with Yamaha Musiccast

Dear all,

I'll try do describe what my issue is, hope someone has an idea what I could improve in the configuration of my UTM/WIFI/AP. I am also not sure if it is a configuration thing with the UTM or if the issue is with above mentioned Yamaha Musiccast. So please don't blame me....I'm troubleshooting and trying to find out where I need to get my hands on. :-)

I have four AP30 connected via LAN with the UTM. The AP's are spread all over the house. All of them are using one SSID (I run them on 2,4 GHZ, channel is auto select). Now to my challenge: also, spread all over the house I have several Yamaha Musiccast devices. All of these devices are connected via the Musiccast App with the SSID and Passphrase my AP's are providing/requiring.

Now as examples, if I sit in the cellar I can't steer the Musiccast devices with the Musiccast App under the roof. Same for other rooms. If I'm under the roof, I am not able to steer other devices like in the living room etc.

I assume that the devices use on one hand the SSID but on the other hand somehow seem to recognise that the SSID is spanned over several APs. I don't know if I should e. g. fix the channel being used or if I need VLAN as an option. Choosing 5GHZ is not an option since not all of the Yamaha devicdes support 5 GHZ. Mesh shouldn't be a topic, since all AP's are connected via LAN to the UTM. 

Hope there is one of you there outside who has a hint for me.

Thanks a lot.

Regards

Damir