The only person to say Paul is an apostle of Jesus Christ in the entire New Testament is Paul himself.
Not true. Read Acts 9:15.
Ananias is told by the Lord that Paul is 'to bear my name before the Gentiles, and kings, and the children of Israel:' This is what an apostle does. This is what Paul did.
Paul is also told that he is to be an apostle by
Jesus. This is what is revealed in Acts 23:11; 'thou hast testified of me in Jerusalem, so must thou bear witness also at Rome.'
Peter also tells his readers that Paul is to be listened to. 2 Peter 3:15,16. 'And account that the long-suffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you; As also in all his epistles, speaking in them of these things: in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction.'