I canceled my subscription due to poor service and the double billing in 1 month without refund issued

I canceled my subscription due to poor service and the double billing in 1 month without refund issued. They came to collect the router and charged R250 for that, which is fine. I paid R500 that was outstanding and that means I owe them the R250 for router collection. I was shocked to see that the R500 I paid was never processed. I have submitted the proof of payment as it appears on my bank statement but I got no help from them. They only communicate via WhatsApp so they just blue ticked me