Private company using my dissertation
I think that your best bet is to contact the IP department at the academic institution where you did your PhD:


They can inform you about your rights concerning the work you did there as a student
Potentially it's also in their interest to get credited for the work done by their PhD students, so they might be your ally in case there is a legal case.


Edit (addition):
Alternatively, you could try to negotiate internally with your company, they might be open to offering you something. But in this case you wouldn't be in a strong position, so it depends how you think the company might react.