DDJ-RB, can't midi map Headphone Cue. External mode needed

Hey guys & girls, I seem to have been stuck on a very simple problem. Due to downsizing my gear (life happend) I chose a DDJ-RB with an SP1 added to it. Seems like a good fit for what I do, and keeps me entertained.

BUT I mix heavily on Headphones mix on the DJM, i rarely use monitor because I have in-ears. And thats one knob that isn't available on the DDJ-RB! I was thinking it wasn't going to be a problem since I have an SP1, and don't use samples, so I could MIDI map the fader in the middle to the HeadphoneMix function, or even the Master Level on the DDJ itself. I can set it on the software once, and never touch it again. 

This doesn't seem possible while the DDJ is connected, it "greys out" the possibility of controlling the headphone mix and level on the software, even the master level... It does give a warning on the bottom, that I should set my controller in External mode. Guess what... also greyed out in the setting menu, can't acces it!

It there a workaround? It is a bit silly that I can't midi map exactly what I want. Coming from Traktor, thats a bit of a culture shock. Heck, in VDJ I can midi map a button to get some food! (no joke, https://www.youtube.com/watch?v=o-7T0_qftaM)

Now I don't need a button for pizza (unless they ring the doorbell the minute I press it) but I would be pleased to map a headphone mix know!

Hopefully you can help!