The B311s-220 has different hardware revisions. Flashing firmware intended for a slightly different hardware version can corrupt the bootloader.
Check Huawei’s official support portal for the newest B311s-220 firmware (non-carrier). Newer versions improve security and LTE stability. huawei b311s-220 unlock firmware
“The official unlock code is $35 per unit,” his contact at Huawei had droned over the phone. “We don’t support grey-market imports.” The B311s-220 has different hardware revisions