@TS, can you give me the exact brand and model of the router? As per my observation with some PLDT routers, bisag eh change and admin password and even the mac filtering, there are still ways to break in. If imo gi-off/disable ang wifi most likely, malware jud tingale na which already resides sa imo PC and once your PC connects to the modem, it alters/modifies the config of your router.