Benefited from this thread, so I would like to contribute back.
DIY Steps to resolve this (i.e. Using M1, want to change a new router but new router cannot get a connection ).
-> Call M1, Dial 1627
-> Choose 2, Broadband services
-> Choose 2, Tech Assistance
-> Choose 5, Not able to connect to internet
-> Need to refresh network, to do a network reset, Press 1
-> Key in your M1 fibre service account number, xxx xxx xxx, Press 1 to confirm
-> Enter Mobile number for M1 to inform (SMS) when done, xxxx xxxx, Press 1 to confirm
-> Power off ONT
-> Power off and disconnect your old router from the ONT
-> Connect your new router to the ONT
<<Wait for SMS, appx 5min>>
-> Power on ONT
-> Power on the new router. It will typically take slightly longer than usual for the new router to get ready.
Done!