how and when can I reach yangjiang from guangzhou?

The last bus for your destination from Guangzhou Bus Station is at about 21:10, so I think you can catch it. You can take the airport shuttle bus line 1 to reach CAAC and then walk to GZ Train where you can find the Bus Station near around.
Or you can take subway line 3 from airport to reach Jiahewanggang and change to line 2 to reach Train Statio and get out of the station from Exit H, then you can see the bus station.