I selected Scale (linear scaling) for the DX1000/DX2000 input range mode. I'm trying to set the upper-limit value to 500 (500.0) but it doesn't let me set a value higher than 300 (300.0).

In the DX1000/DX1000N/DX2000 range settings, the allowable setting range from lower limit to upper limit is -30000 to 30000 (excluding the decimal point). Change the upper or lower limit value or the decimal point to stay within that range. The decimal place is determined by the scale lower limit.
For example, 500.00 is the same as 50000, therefore it is not allowed, but 0.0 to 500.0 is allowed.