You mean you are arriving at Guangzhou airport at 22:00. But the airport provides the airport buses to go to Dongguan from 07:45 to 22:10. I think it's not enough for you to go through the entry inspection. So, you could not catch the final airport bus to Dongguan.
Dongguan is not far from Guangzhou. You can go there by bullet train. By bullet train, it takes around 30 minutes to get to Dongguan.After 22:00, the airport still has the airport buses to go to Guangzhou East Railway Station. But the final bullet train leaves at 22:50 from Guangzhou East Railway Station. Probably, you cannot catch it.
If you don't mind, could you stay one night in Guangzhou? Just find a hotel not far from Guangzhou East Railway Station? Then, on the following day, you may get up early to take the first bullet train to go t Dongguan?? The first train leaves at 06:20.