@bascen 

Old CPE firmware version does not work with new web browser anymore. On your screenshot it's a 2017 firmware.

The solution is to find an old web browser like IE8, or chrome 80 and earlier version. (that can work with TLS 1.0)

  @bascen 

From the screenshot that I see, you can press the "details", accept the risk and open the UI. Have you tried that?

You also mentioned the setup to Router, do you mean you set the CPE as an "AP Router"? In that case, I think you can open the interface when you are connected wirelessly (as the LAN port become WAN, and wireless become the LAN for sharing the internet of physical LAN port).