I got Nodered docker running on my Synology DS220+. Everything was going well, I installed it out of DSM and set the UID 1000 as folder owner afterwards. When I now open Nodered the editor came up but every time says „Connection Lost. Trying to reconnect“ and I can not see any debug messages or active flows. It is possible to build flows an deploy them. I also see that the flows file in the mounted data folder got updatet. But the editor connection is not correct. I tried it with different browser (Safari, Crome, Edge, Firefox) but on all the disconnection is the same.
I don't believe you need to adjust the UID anymore, but to your point:
did you start the container with the --network host option?. If you attached node-red container to the bridge network, I would check if starting on the host network solves your problem.
I changed the settings from bridge to host but it does not fixed my problem. But I then decided to deactivate the port forwarding to the synology web station application and just used the standard settings where Node red is reachable through port 1880. Now everything is fine!
