Return of modem

Level 2
Hi! I recently switched from ADSL2 (never set up) to NBN and I was advised by customer service to return my old modem to obtain a refund. Could you please supply further details about how to return the modem?
My old ADSL2 account number is 674824 and the new NBN account number is 677039.
(Please reply; I have not received a reply to multiple emails and phone calls)