Try different button combinations (WPS + Reset, Power + WPS). Or use a serial console to interrupt the boot sequence by pressing Ctrl+C or Esc in PuTTY.
Since many ZTE routers use Qualcomm chipsets, QFIL is frequently used to flash firmware while the device is in "9008 Download Mode." zte router flash tool
Once the flashing process is complete, reboot your router to apply the changes. Try different button combinations (WPS + Reset, Power + WPS)
Most ZTE routers have a tamper-evident seal. Opening the case or using unofficial tools voids any warranty. zte router flash tool
Flashing a router is NOT like installing a phone app. Here are real dangers: