Ok, I'm probably being thick, so sorry if I am....
If I download Super.SU from the Google play store it just tells me that "The SU binary needs to be updated. Continue?" - On continue it tells me that a custom recovery list TWRP or CWM can be used etc.. then gives me choice of 'Normal' or 'TWRP/CWM'. As I'm not using TWRP/CWM I choose 'normal'. It tries to install but eventually fails with lack of root.
Back to being thick.. My understanding was that SuporeSU requires the device to be 'already; rooted (i.e. Super.SU uses root but does not enable root).
So, when you say just downlaod and run Super SU, are you assuming the Playon (GPD DX) is already rooted? In which case, I can confirm it is not; or at least mine is not.
Should the Playon already be rooted?