I've tried launching the udp broadcast relay service using ssh with the help of the example command at the link you have provided, but it is still not working. I've also tested the config to relay Bonjour udp packets just for testing purpose and I was not able to make it work either. I have attached the troubleshoot logs to the original post. In short the setup is :
-SmartThings Hub behind the RUT240 with LTE failover.
-Multiple Smart Wifi devices that broadcast through SSDP to be discovered by the hub for initial configuration and pairing. (Smart plugs, speakers, smart bulbs, etc.)
At the moment I need to physically connect the SmartThings Hub to the main LAN for it to discover the new smart devices, add the devices, then physically reconnects the Hub behind the RUT240. After de device is added it can communicates with the hub but the traffic between them goes through the internet and not directly from the LAN through the RUT240.
I could keep the config as is since it works most of the time and physically changing the network of the Hub is not too much effort when adding new devices, but it would prevent the Hub to communicate directly with the Wifi devices when internet is down on the main LAN (which does not happened frequently in practice).
Let me know if you find something interesting in the logs and let's try a few more things and if it still does not work after this, I'll probably keep the config as is since I've spent too much time on this lol
Thanks!