On HTC Desire 728 dual sim phone, only first SIM supports 4G/LTE network. The second SIM supports 2G only. Make sure, you insert your preferred 4G/LTE compatible SIM in the first SIM slot and the other you can insert in the second SIM slot. The third slot at the bottom of these two SIMS is reserved for SD memory card. If the SIM tray is missing (since this is an old phone), then while inserting the SIM in the phone, make sure its cut-side is facing up and the golden connecting side facing opposite side of the LCD display screen.
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