That command does work, I only needed part if it …
settings put secure sysui_qs_tiles "wifi,cell,$(settings get secure sysui_qs_tiles)"
… is enough for those tiles to show up:
You need to open the edit menu 
and actually add them to your quicksettings though.
Edit: Ah, nope, they don’t stay in the QS if you move them there 
Adding the first command doesn’t help, they just vanish.
And if you use the command to specifically put wifi among those 6 default QS tiles, all you get is an empty space:
Well, there’s still something missing to get this to stick …