I have Samsung galaxy j2 ( j200h/ds). Phone is supposed to connect to 3g but it never does. My current network is T-Mobile.
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Use OpenSignal to determine if your area actually does have 3G coverage from T-Mobile. I suspect that your area does not, and that is why you are not getting 3G service from T-Mobile. If you do get 3G coverage in your area, removing your SIM card and inserting it back into your phone will likely do the trick.