How to factory reset a ZTE MF266 outdoor router?

Does anyone know how to reset a ZTE MF266 outdoor router? This router doesn't have a physical reset button. Yes, I can remove the back on the router, but there is no physical reset button there. The official method is to log in to the WebUI and reset it through the menu. However, the username/password has been changed, and I can't access it anymore using the default credentials. The documentation says that reset option is available on login screen, but apparently the current firmware version doesn't allow the reset option before login. Here's some documentation for this router .

Question by anibal on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.