Follow the steps -:
• First of all you have to open engineering mode in your device
• After downloading just install Engineering app
• Now app and then select MTK settings > select preferred network option
• Then there you will see list of network there you have to select 4G LTE/WCDMA/GSM & then you have to reboot your device
• Now enter your jio sim in slot 1 and leave slot 2 as empty
• That’s it now you can enjoy 3g network & you will get 3g speed not 4G!!