This is the golden rule of router management. Almost every router produced in the last decade has a sticker affixed to its bottom or side panel.
The primary danger of leaving default credentials unchanged lies in automated attacks. Botnets, such as the infamous Mirai, continuously scan the internet for devices using factory-set username and password pairs. If a ZTE H3601 is left with admin / admin , it can be co-opted into a network of compromised devices used to launch Distributed Denial of Service (DDoS) attacks, intercept traffic, or serve as a pivot point into a home or small business network. Furthermore, default credentials are often publicly documented in online databases and manufacturer manuals, making them the first combination an attacker will try. zte h3601 default password
If still locked out, the flash memory may be corrupted. Contact ZTE support or your ISP for a replacement. This is the golden rule of router management