First of all, make sure that you are actually uploading the firmware file to 192.168.1.1/index.html (NOT /uboot.html).
Secondly, the firmware version that you can be uploaded depends on the bootloader version. Thus, depending on the device you have, you can try uploading the Legacy firmware first, and if it is not accepted, then try uploading a RutOS v7.0+ firmware. For example, 07.02.7, or 07.05.4, or other versions. Firmware for RUT950 can be downloaded from here.