Help! I can't log in to Zyxel router

Hi

I've tried to log in to the router by 192.168.1.1 as it says on the back of the router. I get the log in window but whatever combination of login/password I use I can't log in. admin/1234 doesn't work. I’ve tried to reset the device still no luck. This is important as I can't change the wi-fi password and I suspect that my neighbor is leeching on my wifi as she has my wifi password.
I’m connected to VMG8924-B30A