Disclaimer: Unlocking a router may violate your contract with your original carrier. Always check your local laws and terms of service. This article is for educational purposes.
In the past (roughly pre-2017), many ZTE modems and routers used simpler algorithms. It was possible to find "unlock code calculators" that would generate a 16-digit NCK (Network Control Key) simply by inputting your device's IMEI number. zte router network unlock code 16 digits free