I want to know am I allowed to bring it in without paying any custom or tax fees on it?
Yes you are, however you will be charged Bandarol Tax on the monitor - so if its an all in one iMac then you'll have to pay.
If its not an all in one then sticking it in a box and bringing it over as part of a baggage allowance, or maybe paying a bit of excess on it is the simplest, and safest way to do it.
If it was a PC then the previous advice applies.
JF