WishList Member includes an integration with Plug&Paid. You can set Members to be added to a Membership Level in WishList Member if they purchase a product through Plug&Paid.
First, you can select and enable the Plug&Paid option in the Integrations > Payment Provides section of WishList Member.
The process of connecting WishList Member and Plug&Paid is explained in more detail in the Tutorial Video below.
It also walks through the Next Steps in the process of adding Members to a Level when they purchase a product created in Plug&Paid.