Can't connect a Bluetooth Sub (Gen 3) to Sonos App

If I understand your post properly, you can’t ’add a device’ when connecting via Bluetooth, you must be connecting via LAN, either directly via Ethernet cable, as @106rallye states, or via WiFi. 

you should try to connect it with your router via ethernet.

Thanks! Network Ethernet cable actually helped me to detect Sonos Sub (Gen. 3) via the Sonos App. But another problem appeared right after when I tried to update my App as required by the system to fulfil settings. As a result of repeatedly unsuccessful attempts, I got nothing through the installation. Could you please give me some advice on how to overcome this "installation fail"? 

One thing to try is unplugging all Sonos devices from power, then rebooting your router. Wait a couple of minutes for the router to come back up, then plug back in all the Sonos devices. 

If that doesn’t help, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.