If Paul was in prison for the mystery in Ephesians, and yet we know he was imprisoned prior to Acts 28:28, does this mean that the mystery must have been revealed to him before that time? In Ephesians 6:19-20, Paul asks that prayer for certain things be made, ending up by saying,

19. and for me, that utterance may be given to me, that I may open my mouth boldly to make known the mystery of the gospel, 20. for which I am an ambassador in chains; that in it I may speak boldly, as I ought to speak.

What did Paul mean when he said he was an ambassador in chains? We know that in II Corinthians 5:20, Paul declared of himself and his fellow apostles,

20.  Now then, we are ambassadors for Christ, as though God were pleading through us: we implore you on Christ’s behalf, be reconciled to God.

So Paul was God’s ambassador. Yet now he was an ambassador in chains. What chains? Who had chained him, and why? Paul speaks of himself as a prisoner in Ephesians 3:1.

1. For this reason I, Paul, the prisoner of Christ Jesus for you Gentiles—

He also speaks of himself as a prisoner in Ephesians 4:1.

1. I, therefore, the prisoner of the Lord, beseech you to walk worthy of the calling with which you were called,

 From these passages, it becomes clear that Paul considers himself to be a prisoner of Christ Jesus, a prisoner of the Lord. Since Acts 28:28, he had been bound by chains to Rome. Yet these were not Rome’s chains, but the Lord’s. It was Christ Jesus who wanted Paul right where he was in Rome, not the Roman government. They might have had Paul under house arrest, though since there was no accusation against him or accusers to try him, this may or may not be so. Yet what really held Paul were the chains of the Lord, which were keeping him right where God wanted him to be. Thus, for the sake of the mystery revealed at Acts 28:28, God had bound Paul right where he was. That was how he was chained. It had nothing to do with why he had been arrested by Rome. That, as Paul said in Acts 28:20, had to do not with the mystery of today, but rather with the hope of Israel. Thus, there is no proof in Ephesians 6:19-20 that Paul was imprisoned because he had been preaching the mystery before Acts 28:28. That is just not true, as we can see from reading about why he was imprisoned in the book of Acts.