Can you connect (through the ethernet cable) on both devices?
The "simple" reset (the 5 seconds may vary a little, the indicator that you have to release the reset button is the led that starts flashing) should reset the device to the factory configuration, yes.
But if the devices are functional (i.e. they boot properly and you can access them via Winbox on the ethernet) you could try to reset configuration from command line:
And yes factory pre-paired couples should remain paired after a device reset, but if the devices are functional it is not a problem - if needed - to pair them manually.