AR300M has some built-in driver of USB uart adapters, for example this one:

When plugged into the USB port it will create a device as /dev/ttyUSB0

Yes, you can use serial pins. But there is a lot info will be printed to the serial console by default. This may case a trouble to you.

I don’t have a Cisco routers so I cannot confirm it 100% works.

For AR300M, the serial device is /dev/ttyS0. You can just echo or cat this device.