Are you seeing this recurrence on any and all devices in your home? If so, your router has likely been compromised remotely. To test that possibility, turn the router off. Reset it to the factory defaults. This is typically done by holding in a small, recessed button for 5 seconds or so. Many routers also have a choice for a factory reset in the web pages it displays. So you may not even need to turn it off.
Either way you reset the router, after it has restarted and is ready, try the web again. If the unwanted pages stop, your router was the culprit.