For a modular membership, I would advise you to setup your membership levels so that you can just copy the settings from the previous level and just add the new areas that they have access to.
That way each new month automatically has access to the same content they did the previous month but they also have access to the new material.
So for example, I would setup month 1 and determine what month one is able to access. Then when I create month 2, I would copy the “month 1” membership level and just add the additional access.
Here is where you would do that: