This is really perplexing given that the thermostat is rated to 600 watts and the rhp only 60, and you tested it without thermostat and it got plenty hot. I watched a couple videos on that exact thermostat you are using, there appears to be two separate outlets in it for night and day? There appears to also be a display of either "dimming", "pulse", or "on/off". I would set it on on/off mode if it isn't already and see if that fixes it, and be using the "night" outlet for RHP.