I’m not an expert, but it seems like there could be a couple of problems associated with the network arrangement.
All the Roon stuff (core, remote, endpoints) needs to be on the same subnet. Can you confirm that’s the case?
For test purposes, is there a way you can connect both the mR and the PC running Roon core directly to the router via Ethernet, or else to the same unmanaged switch connected directly to the router?
Also, if @mike or another Roon support person gets back in on this, they’re probably going to want to know details of your setup, as described here.