did you allow boat ferry in your vehicle profile settings? Just checked Calais - Dover with actual OSM on PC and Android - both found about 50 kilometers route using the ferry.
In your car profile you have to enable "Small local roads". It is by default deselected, which already leads to this same question for many, many years.
Not your fault, but an incorrect design decision by MNF (for many, many years :) ).
When I deselect "Small local road", MFN takes me from Dover to Portsmouth by road, ferry from Portsmouth to Le Havre and from there by road to Dunkerque - 690 kilometers in total (fastest).
"Small local road" selected it takes the ferry to Calais and road to Dunkerque (90 km) - isn't there a ferry anymore directly from Dover to Dunkerque?
I didn't find a way to make it take the shuttle train ...
Sometimes it also differs which maps you take. If you take entire UK and Hauts-de-France, or UK South-East with Haute-de-France. MNF sometimes has difficulties with large complex (multi-)polygons which cross borders (administrative boundaries).
I took the last ones. I do not get a route either from Dunkeque to Dover either (or vice versa). It always takes me over Calais.
But so do the other OSM nav apps and Google Maps (but Google Maps also copies from OSM).
But when I try to book a ferry online (multiple options), I can still go from Dunkerque to Dover.
Edit: Even if I select the "Dunkerque Ferry Terminal" as POI as starting point it doesn't work. Strange.
Sometimes it may also differ what device you take ...
OsmAnd (on my Android devices) here shows the Dunkerque-Dover-Ferry on the map (so the ferry is in the map), and after two crashes it really did find a route from Dover to Dunkerque taking that ferry. But planning a route from A to B far away from home is such a shit with that - I can't tell you how I got it ...