The bottom of my freezer has had a lot of ice , it seems like it's coming from some type of back-bottom vent. So i remove it, but now my fridge is not getting cold anymore! This frigerator is only 18 months old! What could this be?

The drain was probably clogged causing the ice. When yyou put the vent back on did you install it properly? If not the air cannot get the ref side from the freezer side. Take it back off and check it

The defrost element or the defrost thermostat could be faulty, causing a build up of ice on the evaporator and blocking the air flow to the fridge. If you defrost the freezer completely and then run it and it cools down in the fridge until the ice builds up again that's your answer.