Modem is dropping internet connection every 30 minutes - Need Urgent Help

I started my internet (1 Gbps) subscription in September. The first month was fine, but since October, I have been experiencing problems with the modem. The internet keeps disconnecting every 30-45 minutes. I have tried restarting the modem and the fiber connection adapter. I even attempted to reset the modem. There is a router connected to the modem, and I tried using a different one, but the problem persists. The issue is not with the Wi-Fi, but with the modem itself, as it keeps dropping the internet connection. I even tested it without a router connected to the modem, but the same issue occurred. This issue is preventing me from working from home, so I need an immediate solution. I tried calling the KPN telephone number, but there was no assistance. Please help!