I was having the very same issue with my DIR-615, hardware E3, firmware v 5.10 -- PROBLEM RESOLVED --
Seems to be a firmware bug - I had both the Admin password and the Personal password set to the same value. Even though I logged in with the Admin user, WAN remote management options were greyed out on nearly all pages. After changing the personal password to something unique the admin functions returned from the WAN. Interestingly enough, LAN management functions were just fine when the passwords were the same.
Scubasnowboarder