If you have a postpaid account with a credit balance over $5 after your cancellation and final bill, Koodo automatically refunds it to your original payment method like your credit card, debit card, or chequing account. If you did not have pre authorized payments set up, they will mail you a cheque instead. This process takes between 7 and 60 days depending on when your second closing invoice generates, so it is usually within 7 days of that bill or up to 2 months total.
Since you are outside of Canada and cannot get callbacks, update your address first if you need to so the cheque goes to the right place. You can just also get a free temporary Canadian number using the TextNow / Fongo app to request a callback through the Koodo site for payment support. They might be able to add a credit card to your account or adjust for a direct refund.
If your balance is under $5 or you have a prepaid account, there is no automatic refund because those credits are forfeited per company policy. Have you already paid your final bill in full? Double check that your account shows the credit clearly. How much is it exactly and is it a postpaid or prepaid account?