I had around >1 month ago similar symptoms of my Lumia not being able to access the internet even though connected to the hotspot.

At that time I put it down to the Lumia being the cause but reading the posts here kinda confirms the N9 may have been the cause...

Funny thing is I just tried it and it works for all my devices... What concerns me however is I got an ad for Joikuspeed; this kinda makes me feel that Joikuspot are disabling their application remotely... If they are suddenly able to remotely send ads they surely retain the ability to disarm their app too hence we are somewhat screwed...