Many Christians understand that Jehovah Witnesses following their church's teachings about Jesus are not Christians - but they often don't know the specifics of what they believe that stand in opposition to what the Bible teaches about salvation, the Gospel, and Who and What Jesus actually is: He is GOD who came to earth to save the world, was brutally killed on a Roman cross, resurrected back to life and appeared to many people as proof, and will come back for His followers! But Jehovah witnesses don't believe Jesus is God, nor that He was Resurrected, etc:
Here's a excellent quick run-down on the Biblically opposing things Jehovah Witnesses are required to be believe and why they believe them: https://www.youtube.com/watch?v=Kox1QCcaAsc
And here's a quick primer on what God requires of us to save us and what we must believe about Jesus: https://discussions.godandscience.org/viewtopic.php?p=253920#p253920