I keep meaning to set something like that up but get stuck on the problem of giving devices on the private network the ability to interact with the guest networ

I keep meaning to set something like that up but get stuck on the problem of giving devices on the private network the ability to interact with the guest network, for example I make pretty heavy use of Spotify Connect for streaming music to speakers, and have Philips Hue lights, both of which need some way of communicating with them from my phone.