First, take a good look at the secondary spider vanes. They are attached to a ring that holds the secondary and the focuser. This is the secondary cage. The far ends of the spider vanes go through the walls of the secondary cage and are fastened with a nut. the vane heads from that nut to the secondary holder and that is attached to the the vane running to the far side. (It may be one piece of metal.) That vane then runs through the wall and is attached by a nut. By loosening the nut on one side while tightening the nut on the other, you can move the secondary back and forth across the front of the scope.
Pull a string or ruler across the front of the scope and you will find that instead of a straight line across your vanes, there is a bend at the secondary holder. That bend is caused by the OTHER set of vanes being off centered. So adjust the nuts on the ends of that one to eliminate the bend.
You our may have to go back and forth on the two axes to get both sets of vanes straight.