Asus basic Config:
Wan connection Type: PPPoE
Enable Wan: Yes
Enable NAT: Yes
Enable UPnP: No
So it isn't acting as a bridge?
I'm guessing the Asus WAN interface may usually be unnumbered when using PPP, but by assigning it a static address within the same subnet as the modem, this also creates a route so the modem becomes reachable.