Power off your speaker(s) and router. Wait a few minutes, and then restart router. When it’s fully started and wifi is up, start your speakers one at a time. 
Then set assigned ip addresses for your speakers.

This fixes a lot of problems. 
If you don’t know how to do the ip stuff, tell us the make and model of the router. 

Make sure the speakers are actually dropping out/off the network, rather than it perhaps being a case of failing to be discovered by the Sonos controller App. When a Sonos speaker loses connection to the network, you will normally see its status LED start to flash, as an indicator that the device is looking to lease an IP address from the local router. If that’s not happening, then check if the speaker has a network connection and an allocated IP address and still responds to button presses, or can be ‘pinged’ over the LAN subnet.

If it is not dropping connection, the likelihood is it’s a ‘device discovery’ issue instead by the controller/App and there are a variety of things that need looking at in that instance. Some things to quickly check is whether the speaker disappears when it is connected to a different WiFi access point, or band, compared to the mobile controller device network connection.

Check to see if there is a VPN client/security software running on the mobile controller that is perhaps preventing discovery of the speaker(s) across the LAN, or see if the mobile is temporarily connecting to a guest network, or even a WiFi extender that does not support multicast broadcast device discovery on the main WiFi segment.

Those are just a couple of things to look for and keep in view, whilst troubleshooting the issue.

Get a smarter router, or correctly configure your existing one.