Share your experience!
Just as Alex don said you can only be connected to a 3G network with one of the SIM cards. Make sure that the network mode is set on WCDMA on the SIM card you wish to have connected to the 3G network. Go to settings > more > mobile networks > network mode.
As the phone is dual standby,you can use 3G only on the first SIM.
Thank You Buddy
Anytime!