|Suggest using the tiniest drop of penetrating oil (such as Plus-gas ,WD40 or Superlube) on the threads and allow to soak a while.|
Do not spray on, instead apply a drop or two with a small screwdriver.
I noted you state screws as opposed to thumbscrews, which appear in Mickliq's picture of monitor connectors.
Then try unscrewing with a screwdriver of the correct size. You should push hardish, while turning anti-clockwise. As advised above, you may have to hold the hexagonal mountings with a spall spanner or pliers to stop them turning.
Bear in mind, using pliers could mess up the head of the screw(s) such that a screwdriver will no longer fit.
Advise back, if successful.
Good Luck - Keep us posted.