Phonos and Chirp don't work on Lumia 640, what could the problem be?

[Windows Central Question]

Phonos and Chirp don't work on Lumia 640

Hi,
I installed my first Sonos speaker today and all works well -- with the exception that in my Lumia 640 Dual-Sim (non LTE) neither Phonos nor Chirp are able to find my Sonos speaker. However I tested as well from Samsung Ativ and Lumia 830 as well as the PC and everything works fine there.

Any idea what the problem could be?

Thanks,
thomas