iPhone (iOS) mistakes my router (mifi) as needing a login page
When you log into any wifi router, immediately thereafter the OS (iOS or MacOS) tries to resolve the DNS for www.apple.com, and then attempts to navigate to this page:

apple.com/library/test/success.html

(Or one of a handful of similar ones distributed around the 'net.) The page is very simple and basically just responds with the word "Success". If they do not get that, or it times out, they assume that you're on a captive router that needs a login. In that case they navigate to the IP of your router/gateway, which in your case brings up the settings, but in a captive system brings up the login page.

Fixing it seems to be a bit of a dark art. Try this solution first and see if it helps.