The best thing for getting waxoil off the underneath is by using more waxoil, put some fresh waxoil on a cloth and rub at the area/areas where you want the old waxoil taken off.

The new waxoil seems to kind of melt/ dissolve the old stuff, keep turning the cloth over and it will come off. Then once all the old stuff is off then you can easily clean it.

Done this to a few cars now and i guarantee you this works.