Looks like for the I51DM model this is the only firmware that will work
http://www.hikvisioneurope.com/portal/portal/Technical%20Materials/00%20%20Network%20Camera/00%20%20Product%20Firmware/E7%20platform%20%281X43G0E%29/IPC_E7S_EN_STD_5.5.111_200908.zip