I need a solution on this topic. How I could avoid this redirection to allow access to my main 4G router admin panel?

  @Kevin_Z 

It's an Archer C6. Yes it is running the latest firmware (until you consider to include bandwidth limit feature we have been asking for years)

Do you consider this as a "correct" behavior?? You are not allowing the possibility of administer another TP-Link device in this case (4G router), it is incredible you are trying to explain how this could be an expected behavior. 

So the only way to access my 4G router if I am connected to Archer C6 wifi is to go and physically connect to it????? Do you actually consider this is something sustainable managing a network??

  @Kevin_Z 

I am having a similar issue with an Oculus Quest 2 headset.  When connecting to the router, the Oculus thinks that the WAN error page is a captive login and keeps redirecting to the router login page. Unfortunately, I don't have a screenshot since the error screen is on the headset. 

Even without that info, in my case I don't need the internet.  I have nothing plugged into the WAN port because, again, I DON'T NEED THE INTERNET.  I am simply using the router as a way to wirelessly connect my VR laptop with the Oculus headset.  The WAN error is completely useless to me and it is necessary that there is a way to disable it.