Unsuccessful SFR resilisation

Hello.

I would like to ask Native French people how to write a letter for the resilisation of SFR.
Actually I have sent the letter and the Modem already at end of January, before coming back
to my home country Japan. I know the process would take 1 months or so, so I don't mind them charging me for the month of January and February...however they keep charging me the fee of March and April.

I have called to the Customer Service before sending the material, and to the correct address he told me.

If you could indicate me the internet site of which I could find the model letter for the demand of "reimboursement" and correct process of resilisation, I appreciate it very much.

Thank you very much!