Can someone help RE Samsung Galaxy S7 Edge dual sim (G935FD) used on the Spark network. Phone starts up on 4G but once start using data switches from 4G/4G+, to H/H+. It then won't switch back to 4G unless restart or toggle airplane mode on/off. Have searched forums, and while some comments are that dual sim causes problems, can't see anything specific.
Spark sim is in the lower slot as upper slot is only 3G or microSD. Phone is parallel import bought from EInfotech on North Shore, who say phone should work fine. Spark helpline didn't seem to know and suggested just turn LTE/4G off, which doesn't address issue.
Is this a known issue? If so is it due to the supported bands/frequencies in the G935FD (compared to single sim S7E), or an issue with Spark 4G? In Balmoral in Auckland so assume coverage should be fine. Thanks.