That instruction was given to the Apostles for their first mission, before Christ's death and Resurrection.
Later the Risen Christ instructs them to go and teach all nations. Matthew 28:19-20
If that's the case.....why then do we see all this surprise and indignation when Peter visits Cornelius? [Acts 10:45] And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost.
And......[Acts 11:1-3] And the apostles and brethren that were in Judaea heard that the Gentiles had also received the word of God. And when Peter was come up to Jerusalem, they that were of the circumcision contended with him, Saying, Thou wentest in to men uncircumcised, and didst eat with them.
This is at least 6 or 7 years after the resurrection. Why are these folks so surprised....Apostles among them? Are you certain that your understanding of [Matthew 28:19-20] is correct?