Free cashback for pay monthly mobile broadband at O2, 1Gb with £60 cash back (topcashback) £20.41
I don't know if you can get more cashback than you expend but take a punt and get at least the free modem and 1 month of usage. Just remember to cancel before the month is up, or longer if you want to justify the TOPCASHBACK claim.
Cancel by email, online chat or phone. 30 days notice required.