Worst Home Wifi ever!!!

I have an MTN home wifi since around June last year. It was ok, not the best because it buffers from time to time. But the past two months has been the worst. The modem switches on and off every 15-20 mins, its almost impossible to work or watch tv with the connection always interrupted. I have called MTN a number of times now but they keep saying the network coverage is good, only the modem needs to be reset. I know the issue is the modem because the previous mobile wifi modem I was using never gave me such issues!!!