unlocked Aircard and various providers

So I've got this air card I bought off ebay, the splash screen says boost but it works with at least 2 providers here in Canada (Bell and Rogers) but I can't get it to work with my provider (Telus) even though the frequencies in the manual match their network.   I also can't get it to work with any of the cheap Walmart pay/go providers.  I haven't tried it with Boost because they don't have as good a deal and the retail stores weren't cooperative.    I've manually updated the apn settings based on the providers websites but the device still says "no SIM".

Any clues why it works with some and not others if it's unlocked?

Thanks,