Ask HN: Past acquisition/partner-interested company trying to trademark our name

We were previously developing a product which had a another well-established company interested in partnering with us to use our technology or even acquiring us.

Talks didn't go through but we showed the other guys our product name, our features and stuff in a pitch deck and other docs.

Fast forward 1-2 monthes, we are gearing up to launch our new product but we've noticed the .com domain of our product name (we didn't own this yet as it was worth $50k and couldn't justify the investment) is bought by that company. However said company is not working on this product and could have only come up with said .com domain name because of seeing our deck. We also know that said company was not developing a similar product or doesn't have the same capabilities. We are now reaching out to said company.

What should we do both legally and in terms of negotiations?

EDIT: This was an issue we didn't really prepare for as the company was introduced by our investors who trusted them.