This post provides Firmware Huawei E3276s-861 21.436.03.00.00 Universal, which can be downloaded from the link which is provided at the end of the post. Once downloaded, extract in a folder using unzip software, and process as mentioned in the respective post. Wrong firmware / software change of a Huawei / ZTE, modem / router, may dead your device permanently. While running the Huawei firmware update, it will prompt for firmware/flash code as password. You can generate the firmware code from universal master code tool.
Disclaimer : Administration of routerunlock shall not be liable in case of unknowing change of the firmware of your modem / router / MiFi. Moreover, if you want any specific firmware of related to Huawei dongle / Pocket MiFi, then leave a comment, if it will be available then will be published. All the firmware which are publicly posted at routerunlock, are free of cost and there is no need to pay anyone.