Drupal Mollie Payment no rules trigger
You need to use event named order_paid_in_full. 

the mollie module does not take of that, it just create the transactions. 
(take a look at payment tab in the order)

So you have two options:


Create you own rule, based on the event above, and grant the user the role.
Use commerce_license module - it has a role module, and this is what you need.