Why my Beam doesn't stay connected on my network

Seems like more information is needed here. Assuming the Beam itself wasn’t moved, it should remain connected to the signal generated by your router.

Have you called Sonos Support to discuss it? 

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

If you have called in, what did they say? That might help give us a direction to go in. 

Provided you aren’t moving your Beam outside… Maybe when you go outside, your phone changes network from wifi to cell. If your phone is no longer on the local network, it can’t see your sonos devices. In that case, it’s not your Beam that disconnects from the network, but your phone.

If you are taking your Beam outside, then it may be losing wifi if the coverage isn’t good there. Why take a Beam outside?