I followed the instructions to install piratebox on the device except for setting up upnp. I can connect to it, but nothing happens when I use my browser.

Did you try to open up the following urls with HTTPS:

Language: PHP
http://192.168.1.1
http://piratebox.lan
http://bbc.com

?

Ok, it worked. However I'm not sure why it doesn't redirect me to the page automatically.

Ok. I assume I’m that your browser is using https per default (or the most time).
For several reasons we do not answer https connection, which is why it’s does not work if your start you browser opening an https page.

Best regards Matthias