One thing is clear your "Raymarine expert" is not interested in solving your problem. You can take his/her number off your speed dial list.
I have a 15 year old C120 and have been running AIS traffic through the NMEA
183 interface at 38,400 B for nearly 10 years. So the interface works, You must have some configuration error. I could try to walk you through a number of issues and still not solve your problem. Maybe someone else has solved
the same problem, but my recommendation is to take your issue to the raymarine forum (Discussion Forum - Raymarine Technical Forum
) where you are more likely to get a solution.