The E8372h-320 uses a "NCK" (Network Control Key) lock. The modem's firmware contains a database of allowed MCC/MNC (Mobile Country Code / Mobile Network Code). If the new SIM's MNC doesn't match the original carrier's, the modem refuses to attach to the LTE network.

Complete Guide to Unlocking the Huawei E8372h-320 LTE Wingle

Because of the enhanced security on the h-320 variant, you have two primary routes to achieve an unlock. Method 1: The Official OEM Unlock Code (Recommended)

Switch to the cheapest data provider in your area.