After a bit more research, I came across the Radiolink website, which suggested I short out the MUX TX8 terminals, with not much other instruction.
So, I decided to just take the plunge and try a little DIY (like everything else on my build) and look for these terminals myself. Turns out on the underside of the APM board, there is a part labelled MUX TX8/OSD, with two terminals running next to it.
I simply grabbed my soldering iron and soldered right across the two terminals
Now that that's done, I'll do some testing and let you guys know how the telemetry on my Radiolink AT9 goes. I'm especially interested in the voltage telemetry.