Ethernet cable not connected

Hey Guys,

Hope you can help. I have recently chnage my ISP to one the using LTE celluar connection. I hoped to use my router instead of the ISP;s one. I have superuser acces to the ISP's router and i can get all of the wlan details. i set up my nighthawk in a simular way but when i try and connect the wlan ISP's cable to the nighthawks internet port, the nighthawk says that there is no cable connected?

In the beginning I had the 2 connected -> internet to the wlan ISP port, lan1 to the the internet port on the nighthawk with both DHCP turn on and worked fine with the 2 networks. I recently changed the nighthawk to an Access point and also work fine. 

its only when i try to but the internet connection straight to the internet port on the nighthawk it says no cable connected.

Please help if you can as i want to use my nighthawk as my main router or if is possible for the nighthawk to connect to this type of network?

also: when i try to login to the nighthawk when connected using ethernet cable using the router IP i get a different login screen that i dont have the username and password for and no way to reset. Image attached.

Thanks in advance!