Standby on Direct Flight when Booked For Layover Flight

I have a flight tomorrow from EWR with one layover to my final destination. I know there are open seats on an earlier direct flight but when I try to go on standby on the app, it does not let me for free.

If I get to the airport early, will they let me go on standby at the gate for free? Or is it not allowed because my booked flight has a layover? The direct flight is 2 hours before my current flight so want to be realistic.