Well thanks for the help guys, but I’m not considering doing changes to the RRA settings / RRD databases.
I can get the total amount of transferred data (using bytes per day) on that specific router interface, I would just need a way to “sum” the total daily bytes, on a date range, and get the total amount of transferred data for that date range… it shouldn’t be to hard 
(but it is, at least for me)
Best regards
Jose