On Playstation it is nearly impossible to arm plastic explosives. Whether the explosive is placed or laid down on the ground alone, or used in an IED. With the combined remote detonation unit, the message to arm the explosive with R2 appears, but after the interaction is complete, the explosive is not armed. The interaction option Hold Arm with R2 still appears, but does not "detonate" with R2.
One guess is that arming does not work near walls and fixed or built structures, but it does in an open field (unsubstantiated).
This bug makes plastic explosives completely useless.