Do you have a heat gauge? Does it rise as the engine warms up? If it isn’t going up, or not much, you have a faulty thermostat and need a new one.
If the engine warms up, but the heater doesn’t, then you probably have air in the heater core and it needs to be bled out. I don’t know how the Jeep heater is laid out, so I can’t help you there.