FNB not supplying 3G data as agreed

I bought a smart device from FNB in May 2014. It came bundled with a Cell C sim and data added based on my Ebucks Rewards level per month for the 2 years I am paying off my laptop. I received my 3G data periodically every month up until April 2015 which was the last data I received. I have been trying to get my data from FNB for the missing months but they pass the blame to Cell C which states the sim was deactivated on 6 Aug 2015 due to no movement via Data or Airtime loads being done on it. So FNB has not loaded any data on my sim from May 2015 to date according to my records. They still try to blame Cell C for the error (I don't care who is at fault, I want the issue fixed). I as the client (FNB client I must add) feel that FNB should sort this matter out and stop trying to get me to do their work for them. I received some reply stating that they will load 500MB of data on my sim next Wednesday (2 Sept 2015) but according to my Ebucks level I should have receive at least 600MB Plus the missing data I should have received in August as per Ebucks level (Which is 2 for August). 800MB is owed and not just 500MB to soothe their conscience. Unacceptable service from FNB.