Typical Synology. Their “opensource package not our problem” copout is not even valid, as they do modify those packages and do introduce issues. Not saying this is the case here but it happened in the past. Before they became too popular they had email support straight to engineering and those folks were brilliant at solving problems. Now the support is pretty useless, making the Synology appliances useless. If users wanted to support them - they would buy something else. /rant.
I skimmed thought the whole thread, I did not see how are you configuring network for running containers, in host mode (not recommended) or with a separate network stack (recommended); in the latter case the MAC address will be different.
Few things that you may or may not have tried
- check that geo restrictions are off on Synology and router. On Synology turn them on and then off, just to ensure the config stays consistent
- remove MAC address from your router forwarding rule. Forward by IP.
- can you access that port on the container from the other local machine?
- start and stop firewall on Synology
- turn off anti-ddos on Synology. It is known to be spooked easily.