I will need to travel to Hyatt Regency Dongguan from Hong Kong and later on Back to HKIA from Hyatt.

So you wish to travel from HK downtown to the hotel? If so, I know Eternal East Cross-Border Coach Company operates direct buses to this hotel. The bus can be taken at Hollywood Plaza (Diamond Hill) at 10:00, Metroplaza (Kwai Fong) at 10:15 or no. 38 of Pitt Street (Yau Ma Tei) at 10:00.

For the back trip, buses leaving for HKIA are scheduled at 06:00, 07:00, 08:00, 09:00, 10:00, 11:00, 14:00, 15:00, 16:00.

I heard there is additional condition to catch the bus. You'd better dial 400-886-1668 to consult the coach company in advance in case.