Converting 32 bit 2's complement to decimal number
Two's complement is a format to describe negative numbers. It applies when the number is negative. Your calculations don't make any sense. https://en.wikipedia.org/wiki/Two%27s_complement

According to RTFM, the data format of the device is 32 bit 2's complement little endian. So given that your MCU is little endian (which is the case with AVR), you don't need to convert anything. Simply store the result in a int32_t.

The data format for UART is LSB first and the tool which made the picture seems to be aware. So if you have some data like 55e70e00, then that's the actual data read. Since it is little endian, it corresponds to hex 0x000ee755, decimal 976725. This is the value you should end up with after reading from the UART rx register. If this number makes sense, I have no idea.

2's complement doesn't matter since the number is positive in this case. A negative number would have a binary 1 in the MSB.