24-hour visa-free transit: Apply in advance or on-site at the airport?

Hi, I have a flight from Warsaw (Poland) to Hanoi (Vietnam) with a 20h45min stopover in Beijing (landing 04:40 in Beijing, departing 01:25 the following day). To return I have another 17h stopover in Beijing (same connection but reverse). Can you please confirm whether I am eligible for the 24-hour free transit in both ways (inbound and outbound flights are separated 1 month)? In that case, do I need to apply in advance? Do I need to notify my chosen airline (Air China)? Or to do that at the airport? Thanks!