I have the same router so understand your question. I think what you are supposed to do is adjust the router depth so the collet sticks out beyond the plate. But mine is semi-permanently installed in a router table so I typically don't do this. Instead, I stick the smaller wrench on the flats and use the walls of the access hole/area as a way to hold the smaller wrench while I turn the larger wrench. You can also get both wrenches on at the same time in the shape of a 'V' and then squeeze both wrenches with you hand.

You can also get recessed/angled wrenches like this that will reach from the top of the router table.