it´s very simple. you can´t just rotate, you need to mirror the leg. mirroring means scaling the leg around the axis it is pointing along. if Z is your front axis and X is the side-axis you have to duplicate with scale with -1 0 0.

make sure you have a clean mesh in the end. after flipping the geometry you perform a freeze transformation. be aware taht this flips the normals to the inside, so perofrm a flip normals operation afterwards. combine and delete history. also have a look into the hypergraph and delete all blind nodes. then export in OBJ format.