K8 Note network signal problem

I used lenovo k8 Note & Airtel (India).

My mobile data always connect.

But my phone not used any inteet when automatically 4g to 2g signal connect. Then needed change network types 4g to 3g or 3g to 4g.

Test :- 1. Safe mode.

2. Factory reset.

3. Network reset.

4. Change SIM slot.

5. Change mobile network (Airtel, Vodafone, Idea)

How to fix it.