Monitor the WAN interface to see what the utilisation is of the link.
So add in the router into NPM and monitor the WAN interface. You will then get lots of info on the link.
Yes, I think the key is that as superfly99 said, monitor the WAN interface in NPM. Since your question is in the NTA forum I think its a problem with trying to use the wrong product. NTA is more about getting granular detail of what traffic is traversing a link, not peaks and averages. NPM however does this quite handily as its all about averaging and figuring out peaks and such.
So, instead of looking at NTA, look at NPM. Since NTA is a module for NPM, you should be able to do that quite easily...