@JSBintheCity,
The Sonos App uses SSDP multicasting to discover the speakers on the network, sometimes (occasionally) I’ve seen this non-discovery issue too, usually with my iPad Pro (3rd gen), but as you mention, closing the App and toggling the WiFi off/on will also sort it for me, usually …then it’s fine for a while.
I don’t get the issue so much with my iPhone XR, nor with my (cheap) Android tablet …so I’m not sure if there’s an issue with some of these iOS devices🤷♂️ - I’ve not (yet) found the cause to this one, but here’s hoping things improve with each new update, from Apple and/or Sonos.
I currently run my Sonos system on a mesh WiFi system. One of the mesh Hubs is my router and the ISP provided router is operating in modem mode. I do have more success with my Sonos system running on SonosNet (one device/Sonos Port, wired to the primary mesh Hub) - that works well for device discovery, but I’m just a sucker for punishment and presently running all on the WiFi signal instead.
I think you are using a Sonos Boost and a SonosNet connection, so you might find that trying a different SonosNet channel will improve things for you.
Its best to set the routers 2.4Ghz Wifi channel to 1, 6 or 11, as they are non-overlapping channels, and also set the channel-width to 20Mhz only to reduce interference.
Then set the SonosNet channel, so it’s at least 5 channels away from your chosen router channel and see if that improves the discovery issue for you. I would also reserve all your Sonos IP addresses in the routers DHCP reservation table, if you know how to do that, as that really helps with these things too.
Note too that ‘WiFi Calling’ and security/vpn software on the mobile controller device can sometimes interfere with Sonos device discovery, it’s ‘sometimes’ slightly more problematic if using wireless extenders, or EOP adapters, as part of the network aswell.
Hopefully something in the above few paragraphs however may help you to resolve the discovery issue.
HTH