[V40 owner 12]

Hi, for weeks now my 98 v40 has been blowing cold air, so I figured the thermostat was sticking, so I bought one at my local car shop but when I removed the old one, the new one does not have the same flat fitted rubber ring around it, it's a round ring, but once fitted although a bit loose, it went together ok once screwed in. Just as before though it's still blowing cold air, also coolant level appears ok and is on the fill line.
As there is 500+ pages on this site I googled the problem and most of the good results brought me to these forums. So I guess after not finding exactly the answer I'm looking for, my question is, is that non official thermostat any good and should I replace it with a proper Volvo one? And also what else could be causing this?? Any help would be great as my kids, wife and me are freezing!