Intelligently Controlling 5 PTAC Units
Page 17 of the manual linked in the question has the information you need to wire up remote thermostats, that is, to connect wall thermostats rather than using the control panel on the unit.  You will have to run five, six, or seven conductor thermostat cable (depending on the unit) from each unit to its thermostat.  

You could follow those instructions to install multiple smart thermostats to control multiple units with the smart thermostat app.  There are also instructions to control two units with a single thermostat (twinning) so you don't necessarily have to install five thermostats to control five units.