Hi,
RUT routers only supports passive PoE, which only allows to supply power to router in it's acceptable voltage range. However router can't share power with other devices: more information about passive PoE you can find in Teltonika wiki: https://wiki.teltonika.lt/view/Passive_Power_over_Ethernet_(PoE).
Both RS232 and RS485 ports can work as data input and output ports. Also, RS232 port in router is configured as DCE device.
Not sure what you planning to do on HQ side, but if you want to have serial communication on both sides, my suggestion would be to use "OverIP" serial mode, then configure one device as server (example: router in HQ) and other as client (example: router in bus). This will create a link between routers and connected devices will be able to communicate with each other over serial.