There is a guide by Roon, either you activate UPNP or you set up a port forwarding rule on your router.
In my case UPNP was already activated but could not access remotely, as the guide suggests I deactivated and deactivated UPNP and now it works. I have read on the Roon support forum that some people prefer the port forwarding option as more secure. But this requires to set up your EX on a fixed IP address
Hope this helps