The Resource Manager is simply what you see here:
Resources | XenForo Community
It has nothing to do with access to forums. Any forums you cannot access is simply because you aren't in a user group that allows you to access them. Customers are automatically placed into the correct user group once they add their forum username to their customer account. We have some custom code that handles those user group changes accordingly.
The Resource Manager is similar to IP.nexus in a few ways, but I don't believe it is a like for like replacement. What features of IP.nexus are you specifically looking for? It's possible they are covered by default functionality, or our Resource Manager add-on, or available as a third party add-on.