For a time frame in Wan Link Manager I see traffic coming from the WAN, it reports ~100GB
For the same time frame I see that Traffic Dashboard reports a lot less. Same in Executive Report
What I can see is that most of the missing reported traffic seems to be from the VPN of a client PC inside the network.
Isn't all traffic shown in the traffic report?
while you are checking that issue specifically, I would suggest you check the accuracy of the other parts of the report.