That suggests that the controller is having trouble maintaining a connection to your speaker through your router, but the Roam is still connected to the internet. 

Unfortunately, you haven’t mentioned what device your controller is on, so it’s hard to give any useful advice. At the very least, I’d recommend a hard reset, but there really is no guaranty there. 

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.

thanks for your reply.

if it helps, the devices where the app was installed were a Fairphone 5 and a Google pixel tablet, both updated to the latest update.

Not offhand, unfortunately, which is why I suggested contacting Sonos support directly. 

Same problem happening again, but this time the suggestion of restarting internet router isn't working. I also reset it the Sonos, but still nothing.

Over the past few months, it regularly happened for Sonos roam to disconnect, and I have to restart Sonos or restart my internet to get it work. All my other devices have strong and stable internet connection.

Does anyone know any permanent fix for this, please?