I'm assuming you're looking for a comprehensive list of default usernames and passwords for ZTE routers, also known as a wordlist. I'll provide some general information and guidance.
Use a pin to hold the button down for 15–20 seconds while the router is powered on. zte router wordlist top
Mastering ZTE Router Security: The Ultimate Guide to Wordlists and WPA Handshakes I'm assuming you're looking for a comprehensive list
MAC-Based: Some older models use a derivation of the last 6 or 8 characters of the MAC Address. zte router wordlist top