Post [Q] Grand 2 Dual Sim Problem

I Have samsung Galaxy Grand 2
I am using 2 Sim cards
one is only for 3G ( Reliance Operator)....& Second is my regular number (Loop MObile Operator)
I hav activated Smart Dual Sim Service

But still When anyone call on my Loop No..its NOt reachable
but reliance works well..

If i turnd of mobile data then both sim works fine..

Why is that..is it bcoz of 3g Network ??
Please suggest me so i can avoid this

Thanks

Priyank Kharote