Oh dang! No, no worries there, router firmware wouldn't have any impact on data usage.
Honestly the only thing that would ever use that much data would be streaming HD quality video, or downloading a lot of data. So if you purchased some HD quality movies from Amazon or iTunes and downloaded them to your computer, or were torrenting a bunch of music or movie or software downloads.
I'm really inclined to think it was an error on your provider's side, though, unless there is some malware in one or more of your computers that's doing some nefarious activity in the background. 2.5g constantly over almost 24 hours is insane.
Have you run a scan for malware? I personally like MalwareBytes. The free version should be totally fine for your purposes:
(you need an account to see links)