There are two different considerations here, which it might be worth separating out.
Firstly, there’s the controller for the fan in your plinth heater, which is measuring the heating flow temperature as a crude way to tell if the heating is on or not. Your heat pump will be running with a flow temperature much lower than a boiler, so the plinth heater never considers the heating to be ‘on’ - and so never runs its fan. This is essentially an electrical switching issue; others posting earlier in this thread have re-wired their plinth heaters to smart plugs or thermostats to control when the fan runs, rather than relying on the heater’s controller detecting a high flow temperature.
(Fancoil units face the same issue - they basically work the same as plinth heaters - so if you search the forums for “fancoil” you’ll find some other threads that discuss this question.)
Secondly, the heat exchanger in your plinth heater will be sized to provide the desired heat output when fed with high-temperature water from a boiler. Its heat output will be much lower when fed with less-high-temperature water from a heat pump. You may find that even with the fan running there’s not enough heat output - but that depends on how you use your utility room, whereabouts it is in the house and how much heating it gets from other rooms or appliances.
In principle, a ‘heat pump compatible’ plinth heater should address both aspects - as long as it is sized correctly for the heat loss from the room.
Was anything said about this heater when (or before) you had the heat pump installed?