Mac Lease change

Hello I have this issue that occurs nearly every day now, it use to be once every month now it's getting more and more frequent. My router would pick up or change it's mac address and I need to either go in and manually change it back to the router MAC which sometimes works or I have to ring ISP to release the connection from whatever devices MAC has replaced it and stopped the internet connection. Please help as I've enjoyed having this router and don't want to go to another option. 

Steps taken: Reset router to factory (3x), Updated Firmware, changed passwords.