AT&T Unlimited LTE Hotspot $20 without AT&T phones

I think the SSN is probably so that they can do a credit check. If you can find someone at ATT who is willing to think, and not just go by the book, you should be able to give them whatever Canada uses for credit checks.

I think there is also the option if getting an SSN as a 'foregner', people do it for taxes and stuff.

Of course the SSN requirement may be some kind of post 9/11 terrorist thing, in which case you may be scr*wed