Ephesians 4:1 I therefore, the prisoner of the Lord, beseech you that ye walk worthy of the vocation wherewith ye are called, ...
Ephesians is one of Paul's 'prison epistles". He wrote them while a prisoner in Rome.
Notice however that he doesn't describe himself as a prisoner of Caesar (though such he was in terms of earthly law).
Rather he describes himself as "the prisoner of the Lord" (not, as the ESV puts it, "a prisoner for the Lord").
Paul as prisoner was awaiting sentence. At the end of his long ministry, he was sentenced to death by beheading (he was a Roman citizen, so he was entitled to a short execution, relatively painless as compared to hanging or crucifixion). But even before the sentence was passed, he was already ready for execution.
He wrote to Timothy:
2 Timothy 4:6 For I am now ready to be offered, and the time of my departure is at hand.
7 I have fought a good fight, I have finished my course, I have kept the faith:
Why was Paul so ready for death?
Because he reckoned himself dead in Christ, dead by crucifixion!
Galatians 2:20 I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me.
He could teach the Romans to reckon themselves dead, because he habitually reckoned himself dead!
Romans 6:11 Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord.
He had accepted God's sentence of death, so he was resigned to being sentenced and executed by Caesar.
Like Jesus, Paul was ready to give up his spirit into the Father's hands. He was at peace, his head bowed before his Father.
We are not the prisoner of the Lord, if we are still demanding our rights, still protesting our innocence, still struggling like the two thieves on Calvary.
Because Paul was submitted to being crucified with Christ, he had already entered into resurrection life and was living in it until his physical death.
What an example!