I think the only option you have left is Airbnb but you have to be very careful about transit. Discovery Bay looks like it has regular bus schedule to the airport terminal and a ferry service to Central but you need to look up the schedule so you don't get stuck. Where exactly is your training taking place? Not all buses travel to the operational side of the airport and there's a chance that you can taxi to those places but can't taxi back out as they don't drive through those areas to pick up passengers.