Another reason for not showing up on FR24 is that they might be filtered out for 'security' reasons. ADS-B Exchange is a less commercial flight tracking site, and they don't filter anything. If it has Mode S, it'll be on there. No ads either.
Is there an app? I'm a very casual user and use it to identify strange aircraft/noises mostly. Such as the very loud an-12 that went over earlier, amazing how much noise they make from 28,000 feet.
Another reason for not showing up on FR24 is that they might be filtered out for 'security' reasons. ADS-B Exchange is a less commercial flight tracking site, and they don't filter anything. If it has Mode S, it'll be on there. No ads either.