Unhappy and frustrated

I canceled my contract in March 2022 and returned the modem to the Constantine Business Park in April 2022. Since I received no further billing, I assumed everything was in order. This week, I find out they have handed me to debit collectors as there is no record that the modem was received. I received no communication from supersonic since cancellation. Now they want me to show them the receipt that I returned the modem or pay. That was two years ago and I know longer have the receipt. This is very frustrating to resolve this matter as I am going in circles