Discovery on separate subnet

I run the Sonos Controller in iOS on my “Data” subnet, and the Sonos Boost and speakers are on a “Sonos” subnet.  It mostly works, except for the initial discovery by the controller of the Boost and speakers.  I’m running a Unifi USG3 router and Unifi switches and APs.  I’ve set up the IGMP Proxy with the Data net upstream and the Sonos net and another “Devices” net as downstream.  The Boost is connected by Ethernet to a switch, and it’s the only Ethernet connection.  There is no firewall blocking from Data to Sonos or Devices.  I’ve set up all of the ports I think are needed from Sonos to Data.  When it is running, everything works smoothly.  If I let the controller run in the background on my iPhone, it comes right up when opened.  However, when I close the controller (swipe up on iPhone) and then try to open it again, it searches and can’t find the speakers.  Turning Wifi off and then on sometimes, but not usually, is enough that when I try to open it again, the controller can find the speakers.  Usually, I have to switch to the Devices network, open the controller, let that fail, switch back to the Data network, open the controller, and it finds the speakers after a moments hesitation.  Usually, but not always, when the discovery fails from the Devices network, the controller “sees” the speakers on another network and advises changing to the other network.  Whether or not I get that message, I invariably am able to switch back to the Data network and the controller finds the speakers.  I’ve tried many things and done a lot of on-line reading to try to solve this, but haven’t found anything that works.  Can anyone help?