I have a 2017 Hyundai Elantra with 118k miles, yesterday it over heated, I noticed the coolant was low so I refilled it, BUT now the heater doesn’t work and the line on the coolant temp gauge occasionally bounces from normal to hot.... AC blows nice and cold but the heater doesn’t not unless I press the gas and even at that, when I’m at idle and I press the gas for heat the rpm either stays were it’s at or it drops... what happening..

 

Just guessing can be expensive and disappointing.
The better way to attack this problem is with a capable scan tool that will provide all the pertinent info for the tech to examine.

Evidently, there was a loss of coolant problem that needs to be addressed
Then if any damage was caused by the engine overheating needs to be checked out.
A good part of the time when RPM needs to be raised to obtain the normal degree of heat , there s a good possibility air is trapped in the system , which is not that difficult to correct, but a good shop will need to tackle these problems one at a time. Unfortunately, it is not as simple as just adding coolant and all will be well.
There is a lot to be said for checking the fluid levels regularly so problems like this can be found before they get out of hand.