Opening a door for someone is a metaphor for giving that person the opportunity to do something. Alternate translation: "God would provide opportunities for us" (See: [[rc://en/ta/man/jit/figs-metaphor]])
Here "in chains" is a metonym for being in prison, though Paul probably also was physically attached to a chain. Paul was in chains because he told people about the mystery of Christ. Alternate translation: "It is for proclaiming the message of Jesus Christ that I am now in prison" (See: [[rc://en/ta/man/jit/figs-metaphor]])