I just noticed, Purism announced the possibility of other modems, back in October.
Just one tiny sentence, easily overlooked 
I donât think it changes much, the âworstâ thing that could happen, is that PureOS has built-in support for four different modems
And one could try all of themâŚ
Check my link above, it clearly lists B2 also, with sources.
But the real question is not how many bands are supported, but rather, do the three supported bands provide a good coverage. And that is likely the case.
@rmccarville, but what makes you think two (or three for Bell) supported bands ar not sufficient? There was a time when dual-band phones were state of the art. One reason why providers use more different frequencies is to support more devices.
I just had a look at some of the references in above mentioned Wikipedia article. In 2014 both Bell and Telus obtained the right to use B17 in most areas, covering 34 of 36 million people. It looks like all three (Bell, Rogers, Telus) have similar coverage in the B4 spectrum (50 to 60 areas each).