What's the default router password on ZTE - ZXHN F660?

Came to my parents place and they said the wifi speed is very slow, so I was just checking the plans and they had 150Mbps plan. In wifi, they're getting less than 60.

Should the router be changed? Also, what is the default router page password for this model? I tried logging using admin, password, blank, none of them worked.

Ah just found it. Password is apparently the MAC address on the backside of the router. Strange., this is the first I'm hearing this.