Well I figured it could be DSL however needed some verification.
I presume this vDSL modem might have an onboard router as well? I've noticed that DSL modems are coming with built in routers now and some cases the router needs to be shut off or made transparent so that the 4500 gets all the data and can do the routing functions. Having both routers going seems to cause NAT issues from my experiences. I would see if your modem has this and if so, can you make it transparent or bridge? The 4500 works better if it gets a full public address. Make sure the Primary and 2ndary DNS addresses are being filled in in the Basic/Internet/Manual settings section. Most ISP can walk you thru getting the DSL modem bridge and setup in the 4500 router. I know, I had to help my buddy with his and he's on ATT standard DSL. What a nightmare that was.