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I will be straightforward enough here and not beat around the bush much. Here is what you can do.
FOR QUALCOMM CHIPSETS ON 3G MOBILE
• Click on Menu (vertical Three Dots on top right side) once the app opens.
• Check out for “Service Menu” or ” Engineering Mode “
• Click on “System App”. Open it and change the specified LTE Bands
Just in case, you don’t get the “System App”,
Go to Engineering Mode and click on it to further change the LTE bands.
• If both these options fail, simply dial *#2263# and select Menu and go back. Further go to select Menu.
• Input key “0000” without quotes. After a few seconds, a popup shall appear.
• Click on UE Settings from that Popup. One done, Go to setting>protocol>NAS>Network Control>Band Selection>LTE Band and select “Band 40”.
• Now simply insert Jio SIM & check for the signals.
• Restart the Device and make sure that you have inserted the Jio SIM in the first slot.
FOR MEDIATEK CHIPSETS ON 3G MOBILE
• Your device information will be shown to you on the screen.
• If a logo of Mediatek Chipset appears on the screen, then your device is definitely using the MediaTek Chipset.
Note: Your JIO SIM needs to be activated before you carry out this procedure.
• Download & Install MTK Engineering Mode app in your mobile.
• Run the advanced Setup from the Engineering Mode/SERVICE MODE.
Note: Make sure you handle this procedure with utmost care.
• Open the installed app, or dial on your mobile the code for Engineering Mode
• Click on the MTK Settings and click on Preferred Network Option.
• Now you can Select 4G LTE/WCDMA/GSM as network mode
• Power off the device.
• Insert your Reliance Jio 4G SIM in 1st Slot.
• Reboot your device for the new network on your mobile.
You will not get the 4G speed; however, your device will run on 3G speed, which is better than having nothing at all.