Remote Login to Asus RT-AC87U Router

When I try to "remotely" login to my ASUS RT-AC87U Router, I'm getting below message and I'm not sure what to do?

Exteal IP: 17x.xxx.xxx.xxx
Inteal IP: 192.168.1.xx

to login I use:

17x.xxx.xxx.xxx:80 

(I'm using port 80 as 8080 gives a "coection failed" error)

but even with port 80, I am receiving below message:

Settings have been updated. Web page will now refresh. Changes have been made to the IP address or port number. You will now be discoected from RT-AC87U. To access the settings of RT-AC87U, recoect to the wireless network and use the updated IP address and port number.

What is needed to overcome this issue please?