Amazon Echo discovery across interfaces

[tre4bax]

I am looking for advice with getting the above working.

I have a wireless network on one interface and a wired network on a second interface.  The Echo I have is on wireless and the server it needs to reach on the wired interface.

There are any any rules between the two interface currently.  If I try to discover then the echo will fail.  However if I plug the wireless router into the wired router rather than the second interface then the echo will correctly find the devices, proving this is about protocol crossing the interfaces.

I have the mDNS relay enabled and on these two interfaces and that does not help.  From investigation I believe this may be something to do with either SSDP or multicast between the interfaces (or both) however I've not been able to figure out what.  Can anyone out there help?