I use Samsung Galaxy A5 (2015) and I have just upgraded the firmware to Android 6.0 (Marshmallow). The phone supports 3G but the option to set it is LTE/3G/2G (auto connect) on Network mode.
How do I get the phone to use 3G only?
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Sign up to join this communityTry to call ##4636## then select Phone/Tablet Information, scroll below and you saw Spinner, select 3G/WCDMA only
Or
Goto Settings > Cellular/Mobile Network > Your 3G SIM > Network (3G only)