Micromax bsnl 3g modem
Net speed and having problem in using sim other than bsnl in it

Iam vry disatisfied with the speed of my micromax bsnl 3g broadband... i have used a 3g bsnl sim in it for months and have never got speed more than 50kbps... i purchased it for Rs 3000 and i was told that all sims can be used in it but now when i put sim other than bsnl it shows invalid sim and asks for a pin code what should i do??? plz reply soon