But as soon as I disconnect the cable after setting WDS up, my master never finds the slave, I put both routers into Friendly Firmware just to be sure I wasn't doing something wrong in the Professional firmware.
If that doesn't work, is my only option is to have it as a wired repeater?
You have to be doing something wrong. Call support and have them walk you through it. You can also use one of the FAQ's pinned to the top of the wireless forums.