Thank you very much for the clear instruction!!!
However,, if I put the value 180 as shown in the screenshot, the flag is in diagonal, so what I did was copy the original value displayed on Rotation "X" upon opening the DAT file with S3D (3.141593) to Rotation "Z" instead of 180.
That solved the problem. Anyway, thank you very much AlexB since you showed us the solution to this problem!