We have checked product with the default theme and we were able to add to cart even with the default theme https://gyazo.com/be6340cd0cfc6b887a935649113d0e87
The only difference was the cursor and button color. I have added the same styles as the default theme has.
So, contact the plugin author about the problem.
One standard license is valid only for 1 project. Running multiple projects on a single license is a copyright violation.