Hi @etienne,
The MAVLink Endpoints are specifically for MAVLink communications, so are unrelated here.
The Serial Bridges functionality should work like Companion’s Routing in recent beta releases. If you’re on 1.0.1 stable or an early 1.1.0 beta release please try a more recent version.
If you’re already on a recent beta, are you getting any notifications about things not working? It may be helpful if you can provide a log of the service.