when i move from 2g connection to 3g my iphone xr is not automatically switching to 3g. it continue to be in 2g. when i restart then it is connecting to 3

Hi sugaan,

Thank you for posting in the Apple Support Communities. I understand that your iPhone doesn’t automatically connect to a 3G network unless it’s restarted. I’d like to share some steps with you that may help. 

First, make sure that your device is backed up to iTunes or iCloud and fully up-to-date: Update your iPhone, iPad, or iPod touch

If your iPhone is already updated, be sure to also check for a carrier settings update. Here’s how: Update your carrier settings on your iPhone or iPad

Lastly, I suggest resetting your network settings. This resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before. Of course, be sure your data is backed up, then go to Settings > General > Reset > Reset Network Settings. 

I hope this gets you started. Kind regards.

Where are you and who is your carrier that a 2G network even exists any more?

Most carriers are completely shutting down their 2G networks.

What model phone do you have, exactly, and where did you get it?