Router/Modem Port Forwarding Problems, Please Help!

[dropGG]

Hey guys, so my internet is running through my modem that my ISP installed which is a Arris DG1670A. My router that I purchased is an Asus RT-AC66R, this connects to the modem. I don't know if this is the correct way to set this up, I just did what I thought was correct and I've been able to have my internet running just fine with the two.

 

Now here's the problem, I can't portforward.. I tried portforwarding on the Arris, but everytime I enter my ip on the configuration web page, it says that it has an invalid IP Address.

I also tried port forwarding on the Asus, which seems to go smoothly, but once everything is set up.. the ports don't actually work. No one can connect to my server. Is there anything I can do to fix this?

I have tried numerous factory resets of BOTH devices, I don't know what else to do. Thanks for your help!