It’s probably going to be something like from here, regarding port forwarding:
https://www.synology.com/en-global/knowledgebase/SRM/help/SRM/RouterApp/internet_port#t2
Traffic hits your router, needs to get sent to your host synology (port forwarding on the router).
Traffic hits your host machine, needs to get sent to your docker container (port forwarding on the synology).