Media skip/play/record/pause/etc buttons are all horizontal. The literal meaning of the one olf proposed is "skip forward" when horizontal.

But our uplink/downlink symbol is vertical and imo giving it a new meaning by blocking the up/downlink. Also the effect of "change the icon in place" when selecting another profil will contribute to the user understanding its meaning. What will happen when you go eg. from online to offline is that the trails disappear, the arrow-triangles stay in place and get blocked by the outer streaks.

Yes. In sailfish icons the highlighted areas suggest "activity" to me. So i think of the online/mixed icons showing activity on the up-line, the offline icon is actively "blocking".

Not quoting all because i fully agree.
I proposed a long-press for this also some pages back but now that rinigus mentioned to make the 2 functions of the buttons available by cycling through them it even makes more sense to me.
That brings to mind we are switching states and that could be made clear to the user by having three or two distinctly designed states of the buttons in question.
For consistency reasons i would keep the white(or black) background button part unchanged but signal the current state whithin the icon/symbol.
Cycling through features is always better to understand when the thing you click on reflects the state you are currently in.
Also +1 for notifications or/and additional "A" rinigus mentioned to make it ultimately clear.

@Fellfroschs red dot idea is along thoses lines whereas i think changing the white/black shadowy backgroundbutton will do no good.