After replacing the any part of the cooling system (any part that has coolant in it, like hoses, thermostat, radiator, water pump) you must bleed the air out from the system.
Luckily, it is an easy task to do.
Using an 10mm Allen wrench (aka Hex wrench) open the bolt that circled in the picture below.
Start the engine and let it work till only coolant will spill and no air/bubbles will go out of the hole.
Then, while the engine still works, screw the hex bolt back in place, tight it only after the engine is cold (do not tight while the engine is hot, you may ruin the housing/screw).
Add coolant if needed during the bleeding process and verify it is on the right level when you are done (add if needed)
If from some reason you need a new plug, here is how to find it:
It is called “mopar hex plug” part number: 6034331
It is a 3/8-18NPTx.475 screw.