I could be wrong, but I doubt it is any thing to do with this.

Does your contract with your network operator allow you to tether?
They most likely do not, and so is the reason you are being redirected to a sign up page to pay extra for tethering support.

It takes time and resources for some network operators, like mine, to detect that you are tethering, once they have flagged you as potentially doing so they do more frequent checks on your internet activity.

Most commonly they check the browsers user agent, but the most effective way is by looking at the Time To Live (TTL) field of the packets; if you are tethering then this field will be +1 more than it should other wise be.

Try spoofing your desktop browsers user agent to that of the N9's stock browser, and by increasing the TTL as described here for Windows users, or for Linux/MAC simply, changing the value within this file: nano /proc/sys/net/ipv4/ip_default_ttl to one greater, and then retry tethering.