Why there is only dual network mode option in lumia phones?

[Windows Central Question]

In network setting there is only one option that is to select the highest speed the network may provide i.e. 2G 3G 4G. If we select 3G then the phone works on 3G as long as the signals are very strong, but the time 3G signals become a bit weak the phone switches to 2G the very moment. This makes browsing and downloading on lumia awful. Please help if there is any way to select 3G only mode.