This post provides Huawei E5573s-156 Firmware 21.180.99.07.00 General, which can be downloaded from the link 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 a password. You can generate the firmware code from the universal master code tool.
Disclaimer: Administration of routerunlock shall not be liable if you unknowingly change your modem/router / MiFi firmware. Moreover, if you want any specific firmware related to Huawei dongle / Pocket MiFi, leave a comment; it will be published if it is available. All the firmware publicly posted at routerunlock is free of cost, and there is no need to pay anyone.