In a vehicle there are often no WiFi signals available from the outside. So you have to start your phone or tablet in AP mode and then start AP mode on the 536 and then switch your phone or tablet back to infrastructure mode and connect to the 536 WiFi signal.
It's a mystery why the WiFi software in the 535 needs to see another WiFi signal before it can start it's own AP mode. Maybe it scans what channels are occupied and tries to choose a free one to use, but the code haven't been written to anticipate when there are no WiFi signals in the air? A manual selection of the channel would have solved that and also made it possible to solve any local problems you might have from certain neighbouring channels.
/Ubbe