Welcome to the forum, 'Caddy3'.
My first thought is the navigation settings menu. Have you got anything there selected (or not selected) that might prevent the system from using motorways?
I can't remember, sitting here without access to the information, what the various settings are but you can check for yourself, if you haven't already. Is this a new problem? Is the car new to you? Is the software original or has it been updated? Although I'm not familiar with your model, I imagine it is probably equipped with the 'Here', formerly 'Navteq', satnav system and is too old to be equipped with Android Auto or Apple CarPlay. That said, it should pick up motorways ok so you need to check the settings first.
Beyond that, perhaps another forum member, familiar with your model, may be able to shed light on the matter.