In Isaiah God says there is no Saviour beside Him ..
Isaiah 43:11 KJV
I, even I, am the Lord ; and beside me there is no saviour.
The NWT states that Jehovah is that Saviour and Jehovah says there is no other Saviour beside Him.
So, how can the Lord Jesus be the Saviour?
When you accept that there are more than one person in the Godhead - you will appreciate that The Lord Jesus is the God of the Old and New Testament.
Jesus and the Father are not the same person, both unique, but the are the same God.
The Jehovah's Witnesses have their minds conditioned to believe that God is one person, this creates confusion towards themselves.
The Bible tells us in Numbers 21 that the Lord God who was and is the Saviour of Israel, chastened Israel because of their murmuring and tempting God.
Paul reminds us in 1 Corinthians 10 that Christ was that Saviour who chastened Israel.
1 Corinthians 10:8-10 KJV
Neither let us commit fornication, as some of them committed, and fell in one day three and twenty thousand.  Neither let us tempt Christ, as some of them also tempted, and were destroyed of serpents.  Neither murmur ye, as some of them also murmured, and were destroyed of the destroyer.