Keep holding the Reset button until the Power LED starts blinking rapidly or changes color, indicating it has entered the bootloader recovery state.
Log into the Web UI (using your administrator credentials) and manually set the 2.4 GHz and 5 GHz channels to ones that are least crowded in your area. Zte F670l V9.0 Firmware Fixed
Launch Tftpd64 and select your static IP in the "Server interfaces" dropdown. Keep holding the Reset button until the Power