Do you have multiple wireless access points within the local network system? - Are they operating on a different WiFi channel? - the App communicates and discovers the players using SSDP multicast over UDP and sometimes that upnp protocol might not be working correctly on the local network.

It might just be the router, so perhaps see if there are any firmware updates for the router/access points and see if that fixes the issue.

A further issue could be the mobile controller device automatically switching between access points and/or guest WiFi networks linked to the same subnet, so perhaps switch off any guest networks too if that is applicable here.

Hello Ken, 

Thanks for the first recommendations.  Unfortunately none of this recommendations applies to my system.  I checked the router and also I set a fixed WiFi channel.  One of 2 devices I could connect with the App and it worked.  After one day I had the same effect as before.  It is strange that the last streaming service can be switched on and off on the device.  The app is simply a disaster and unreasonable.

I don’t personally have any issues with either the S1or S2 App. Which router make/model is it?
If the router perhaps has an ‘Airtime Fairness’ feature, then try switching that off.