It is a big issue in most of 4g smartphonesthat these phones don't have the feature of 4g only or 3g only switch mode .

But don't worry here is a solution for this problem

1:download 4g lte Dual app clicking here
Download Now

2:click on 4g sim one

3:select your desired network
From their
Wcdma only for 3g only
Lte only for 4g only
Gsm for 2g
Your problem is solved now enjoy