Firmware username and password

i received this router form my ISP but the firmware on router now It doesn't work well and I've seen the original Zyxel firmware for this router, there's a lot of features that don't exist .

I know Zexel is a big company with excellent customer service.

my question is if I downloaded firmware form website Zyxel version 5.50ABPM8.3C0

the default username and password on the device label work on version 5.50ABPM8.3C0 or will change.

if it change what the username and password

thanks.