Be careful
The divorce laws in the DR are different than those in Canada so be careful what advice you follow.
A DR lawyer can explain to you the laws and property division rules there, but Dr. Guzman has already put that info on the DR1 board, and you can search for it. Generally, if everything you owned before and after the marriage would be divided except real property (real estate). This includes half the debt. This is a broad summary so get advice from someone qualified (not me).
In Canada, the laws depend on which province you live in. In some provinces, he would be entitled to half of everything you acquired since the marriage. This includes half the debt. If your house was paid for before the marriage, he might not be entitled to any of it if the agreement he signed stipulated that. However, he might be able to make a case for half of the change in value over the life of your marriage. If you cannot enforce the pre-nuptual agreement, then he has dower rights to the matrimonial home and he would be entitled to half. Again, consult a professional.
As to the validity of the marriage.... if he was legally married to another woman when he married you, your marriage is not valid. You can get an anulment on the grounds of fraud, and this should help your property settlement case. If you want to go so far, fraud is grounds for deportation since he entered Canada on fraudulently. That should stop him from taking his Dominican wife and children to Canada and possibly living in the same community as you since that is where he has contacts and is established.
As to the question of the validity of the agreement he signed. For it to be binding in Canada, he had to have independent legal advice. It sounds like he did, but since you don't have the guy's name, it could be hard to prove. For it to be binding in the DR, I think that it had to be registered in the DR at the time of the marriage. Dr. Guzman could advise on that issue. But since you have been living in Canada, you are probably best advised to pursue dissolution of your marriage there.