Hello,
In my case, my ActionTec router was connected via a coaxial cable, so I was unable to directly plug in my DGL-4500. What I did was turn the ActionTec router into a bridge and that works perfectly. My DGL maintains the WAN IP and other configuration settings just as if the ActionTec was never there to begin with.
You can do some Googling on this, but here is a guide to get you started on setting up the ActionTec as a bridge if you want to go that route:
http://www.dslreports.com/forum/r19559467-How-To-MI424WR-Network-Bridge-working-FIOS-TV