First you must check your ZTE MF90 Mobile WiFi router to have at least 30% charged. If not, charge it.
Just insert a non-acceptable Network SIM (i.e. another Network SIM) on your ZTE MF90 router SIM card slot. (Now it shows you message like SIM lock (or) no signal (or) Invalid SIM).
After changing the another network providers sim on your WiFi/MiFi, switch the device ON. Don't panic because you are using another network providers SIM.
Now just establish a WiFi connection to ZTE MF90 WiFi/MiFi router with your PC / IPAD / IPHONE / Android Device.
Connect your ZTE MF90 with PC and login to your device’s dashboard at 192.168.1.1/simlock.aspOR192.168.1.1 OR 192.168.8.1
Go to Advanced Settings –> SIM Settings –> Unlock Device –> Enter Unlock Code –> Click on Apply. OR Open your Settings –> Dial-up –> Unlock SIM card –> Enter 8 digits unlock code (NCK Code) in your ZTE MF90 WiFi MiFi router.
Click OK then the device will reboot to display the new network.