Serial connection to UPS from routerboards is a little tricky.
Here is what you need to do.
First you need to disable the serial console.
/system console print
Flags: X - disabled, U - used, F - free
# PORT TERM
0 F serial0
/system console disable 0
Now you can set the UPS to use serial0, you can only have a serial port doing 1 task, so that is why winbox complained.
Next there is a bios/boot loader setting you need to change.
In the boot loader configuration you need to change the 'boot key' setting
It defaults to 'any key' , change it to 'Delete key'
The boot key change is needed because the UPS will be sending information over the serial cable and it can interupt the boot process.
Due to the extra setup needed and that you no longer have the serial console to login when the network ports fail, I actually recommend you not use UPS on routerboards. If this is a x86/PC router and you have an extra Serial port, then go ahead and use it.