Prisoners were often bound in chains. Paul was in prison when he taught Onesimus and was still in prison when he wrote this letter. Alternate translation: "while I am in prison" (See: [[rc://en/ta/man/jit/figs-metonymy]])
Paul was in prison because he preached the gospel publicly. This can be stated explicitly. Alternate translation: "because I preach the gospel" (See: [[rc://en/ta/man/jit/figs-explicit]])