I have two WANs.

On the LAN side I have the ER605 serving IP addresses (192.168.0.xx) to a number of devices.  One of these is a Mesh network, which has its own router and subnet (192.168.5.xx).  I have a number of devices on the mesh network - one is a small Home Assistant server, which uses duckdns to provide a dynamic DNS service.  I have set up the server's static IP address etc. and all the port forwarding necessary from the WAN side to (half the time) connect to the HA server.

But sometimes I cannot access the server from the WAN side using the dynamic DNS service.  I think this is because the server is reporting its WAN side IP address on both of the WAN connections, as the ER605 routes over each of the load balanced WAN connections.  So sometimes the Dynamic DNS service is significantly behind the curve.

The HA server has to be on its own mesh network as I have many devices, like sensors etc. on the same network, which the HA server discovers and interacts with.

My ideal solution would be to tell the ER605 always to route outgoing connections from the HA server to one particular WAN, but not route the whole mesh network (as there are streaming devices also on the mesh network) to the same single WAN.  The HA server has a static address on the mesh network (let's say it is 192.168.5.100) and I can also get its MAC if necessary.

Is there anyway I can tell the ER605 to always route traffic from 192.168.5.100, or the particular MAC address over a particular WAN?