R7000 + Weird Problem

I just picked up a R7000 to replace an old DIR-655 Dlink for our corporate guest internet. It's run off of it's own internet connection. Completely seperate from our LAN internet ISP

Here's the weird stuff. I have internet on the R7000, everything works....almost. If I use a comptuer on this guest network and access one of our websites that is hosted internally it doesn't work (any serverice hosted internally wont work, over wifi or the ethernet ports). Just straight internet connection to our corporate LAN router - DNAT to the internal server. I can see the traffic on the corporate LAN router, nothing is being blocked. If I throw the Dlink DIR-655 back it works again. If I take the R7000 out of the loop, and plug direfctly into the seperate ISP modem, it works again. Somerthing specific to the R7000.

Doesn't make sense, it is totally seperate internet. I might as well be accessing our stuff from my home internet. Everywhere else works, except throught the R7000. 

WTF?