I got a similar behavior with my last Router I had (it was a DLINK Router), there was an option "do not Display Router Configuration Page for WiFi Devices".
This is just a possible option I can imagine now.
Opera Mini would generally not work, since it's traffic is routed over the Operas Servers to compress the Webtraffic that is been sent to your smartphone.
Unlike straightforward web browsers, Opera Mini fetches all content through a proxy server and reformats web pages into a format more suitable for small screens. A page is compressed, then delivered to the phone in a markup language called OBML (Opera Binary Markup Language), which Opera Mini can interpret. The data compression makes transfer time about two to three times faster, and the pre-processing improves the display of web pages not designed for small screens.
Source : http://en.wikipedia.org/wiki/Opera_Mini
And since Opera isn't able to route into your private network it would not work.