sonos app keeps losing my speakers

Look in the router settings for something called AP isolation or client isolation. If it's on, devices on the wifi can't see each other, and the app loses the speakers exactly like this. Some routers turn it on for the 5GHz band only, which explains why it's on and off. Update the speakers and the app together as well, a mismatch between the two caused the same thing for a lot of people.

iPhone? Check Settings > Privacy & Security > Local Network and make sure Sonos is on there. If it gets switched off the app can't find anything. After that it's nearly always the network. Phone and speakers have to be on the same network, so a guest wifi, a VPN on the phone, or a mesh system putting devices on different bands can all break it. Giving the speakers fixed addresses in the router (DHCP reservation) helped mine stop vanishing.