I am using core/remote on different subnets in my setup. The trick is to run “udp-proxy-2020” on your router.
My setup is that two subnets are routed by an OPNsense machine. The udp-proxy-2020 will send the needed information to the other subnets.
You may read these threads in tinkering category.
https://community.roonlabs.com/t/roon-in-different-subnets/199763/2