how can I turn off 3g and turn on 2g in ios 11.3?

Hello полина187

To enable 2G networking you have to follow these steps:

Settings -> Mobile Data -> Mobile Data Options -> Voice -> Check 2G

I hope I was able to help you.

Have a nice day

Marcel

2G disable option has been pushed from Apple during iOS 11.3 update for iPhone 6 and above Without prior notificactions.

2g toggle option is available for iPhone 5S Users.

Apple should consider to enable this 2G option during their next update for all iPhone users.

I personally prefer using 2G signals when I am at home and work (Use personal Wifi for internet) this will enhance the iPhone battery life and iPhone performance