The first suspect would be your thermostat is stuck open, or failed, assuming you have already confirmed the fluid levels are fine.
On another vehicle (non-hyundai) I had an air lock in the heater core that would send no cabin heat, regardless of the engine temp, but that's not your issue, as you said the gauge shows the engine is running cold.
As a check on the thermostat you can try putting cardboard in front of the rad. But really watch your heat gauge if you do that! I've never attempted this on a Hyundai, so not even aware if there is space to do so.