I only use it when I’m not on my wifi network at home. Outside my home Roon doesn’t work unfortunately. The, rather complicated, workaround is to use a VPN to connect to my home network, enable squeezebox streaming on my Roon core, start iPeng (a squeezebox client), and then I can use Roon on my phone to play the music to iPeng (pfff… 
). I’m still hoping Roon will lift the restriction that the endpoints need to be on the same network. Then I won’t have a need for iPeng anymore.
(see also VPN: step-by-step)