We went through that round of things previously. They are all on the same subnet, I've never set up any separate subnets for anything, I wouldn't even know how to. Out of three hubs the C8 (which actually has all of my physical, noncloud devices, mostly Zwave) and a C7 (which has my cloud devices, but I only tried it as an experiment) are not discoverable by Home kit. Only one C7, the one that runs all of the automation, actually will regularly connect. I've even gone so far as to unplug the C8 and plug it into the same port as the C& which Does connect, and even by-passing the POE switch and plugging into the router directly, I have never been able to get it to connect to home kit.
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