Now put another SIM in your modem and connect with your laptop/PC. It’s will ask for unlock code. You have already got unlock code for your modem through Universal master code. Don’t use, because it will not accept. This software generates unlock codes for 35 series IMEI number, but if will see your modem inside you will get 86 series IMEI number.
Airtel has recently launched Huawei modem with the latest custom firmware 21.157.13.00.284, the Huawei E1731. Above the dongle, it is printed as E1731, but inside the modem, you will see Huawei E173cs-1. Huawei E173Cs-1 firmware is totally different from previous one firmware of Airtel E1731 (E173Bu-1). Before this Airtel had launched modem with custom firmware version 11.126.16.04.284 which is not unlocked in the market. Hardware version of Airtel E173Cs-1 is CH2E303SM. Firmware version 21.157.13.00.910.B757 works perfectly with E173Cs-1 data card and can be unlocked. When you flash then it will ask you for a flash code. You can download Universal master code and just run this.
Put your modem IMEI Number in the appropriate box, and just click on Calculate button. Now you will get Unlock code and Flash code. When you will flash your modem it will ask for a code, then provide flash code.