How to use 4G sim in 3G or 2G phone

Note- Before continuing through the process you will have to ensure that your phone has MEDIATEK chip inserted. If you do't know what is MEDIATEK chip and how to check it then follow these steps-

1. Go to Google Play Store
2. Search for CPU-Z app and download it.
3.Open the app and at top if it shows MEDIATEK then this means    that your mobile phone is supported by MEDIATEK CHIPSET.
4.If it shows anything else rather than mediatek then this means        your phone is not supported by mediatek chipset.

Coming back to the Process

Now that we know that if our device supports mediatek or not we must proceed to our next steps to use 4G sim in 3G or 2G phone.

To use 4G sim in 3G or 2G phone follow these steps-

1. Go to Google Play Store and download an app called MTK     Engineering Mode   

2. Open the app Go to 'Android Setting' then tap on Phone Information  
3. After opening up the Phone information setting scroll down until you see the Set Preferred Network Type.
4. Then under this you will get a dropdown menu, tap on the dropdown menu and select the option named LTE/GSM/CDMA auto (PRL).
5. Then reboot your device and install a 4G sim in it, the 4G sim will automatically start working.