When you buy a ticket from these sites, you are not buying a ticket from the airline. You are buying a ticket from the site. The site promises to buy a certain number of tickets from the airline at a much lower price than you can get. They then resell them at a higher price. The difference is their profit.
Brussels Airlines customer is Kiwi.com and not you. They don't have to answer your questions because you didn't purchase the ticket. Kiwi.com did.
Without sounding like a broken record, always buy airline tickets directly from the airline or from a licensed travel agent. However, many see the "wow, that's a cheap price for that flight" come on and buy from the third party. If everything goes right, you've saved money. But if there is a problem, you lose.
I highly doubt the airline did anything wrong. They can't legally cancel your ticket so they can resell it for a higher price. For most airlines, all it takes is the passenger name and record locater to access the flight records and cancel the flight.
Chalk it up to an expensive learning experience. You're not the first nor will you be the last.