Some more detailed feedback for those potentially facing similar issue:
as suspected and highlighted by Derelict, problem was misalignment between DSL device and pfSense. In order to reach internal web service, if DSL device acts as a router, 2-steps NAT is required. One from internet to pfSense and one from pfSene to internal server.
This needs to be consistent all along the path however paying attention not to open everything in order to grant access ;)