Nighthawk AC1900 Not Recognizing Signal from Modem

I recently upgraded my internet to Gigabit service. As a part of the install, the ISP provided a second modem for me to use. Essentially this allows me to either use the Gigabit service or use the 60 Mbps internet I had before. When I use the old modem and the slower service, everything works great (I'm using it now). However, when I use the new modem with the faster service, there is a very fast signal when I plug the Ethernet cable into my computer directly, but the internet light on my router stays amber. So far I have downloaded the latest firmware, reset to factory settings, and reset each individual piece of hardware. The ISP has said that it's a router issue and that I need to go buy a new router. I'm really hoping that's not the case. Any ideas?