The Local Area Connection (the NIC) is the one to look at.
If a RRAS server is directly connected to the Internet, the public NIC
is referred to as the external interface and the private one as the internal
interface. This can be confusing, because the "virtual" interface which
remote clients use is also referred to as the internal interface (that is
the one you see as internal in the RRAS console).
Since you are behind a router, you do not have an external (public)
interface. The router does that job for you. The setup wizard sometimes gets
confused and sets up filters on the NIC, blocking non-VPN traffic.