How to use Jio 4G on a WCDMA handset?

Practically speaking, there isn't a possibility of accessing 4G connectivity on a 3G phone. You will need to buy a new 4G enabled device for the purpose. Ensure that the phone you buy supports VoLTE.
However, there are some tricks being circulated over the internet about some possibilities about being able to access the same on a 3G smartphone. Though I would not terms them as reliable, you may try the tricks at your own risk.

Method 1
This method works on Qualcomm chipsets. Please ensure you are running Android Kitkat 4.4.

Steps involved are -

1. Download the Shortcut master app. You can get it from the link https://play.google.com/store/apps/details?id=org.vndnguyen.shortcutmaster.lite&hl=en.

2. After launching the app, tap on Options.

3. Locate Engineering Mode or Service Menu.

4. Under it, choose Change LTE Bands.

5. Change the band to Band 40.

That should do it. However, please note that I have not used this method. It has the risk of making your device unusable. Proceed at your own risk.

There are some of the tweaks that helps the JIO 4G to be run under WCDMA devices. Do note that most of the tweaks out there depends on the android version. At the minimum the device needs to have the Kitkat version. You may notice that the method for the Marshmallow may be different too. Also the processor type makes a lot of difference. The below method works with the Mediatek processor.

Here are some of the steps that you can follow:

1. Install MTK Engineering Mode app on your phone.
2. Open the app in engineering mode or service mode.
3. Check the MTK Setting tab.
4. Choose the Preferred Network Option.
5. Select 4G LTE/WCDMA/GSM as network mode.
6. Save the settings and shut down your device.
7. Insert the Reliance Jio 4G sim in one slot and keep the second slot empty.
8. Switch ON the device.

By doing the above steps you'll be able to see the Jio 4G on your device. There is another method for the Snapdragon processor and with different Android version. But that depends on the phone and android version.