Secondary Port, randomized each start up and after being “offline” for too long.
Random Port for UPnP
If you can allow incoming and outgoing 9993 and incoming return traffic, you may have some luck.
For best results, a device needs be able to send to any address, on any UDP port to directly connect with other devices.
If the NAT type is “symmetric” or “strict”, each vendor uses different terminology, it will be difficult to make direct connections. Look for Full Cone NAT, options related to VoIP, persistent NAT, etc…
Ask your vendor. Let us know what works.