The mystery you quote from Romans is found in the Scriptures of the prophets - Rom 16:26. It is a part of the Mystery of Christ - Rom 16:25. The Mystery in Colossians 1:26 was hid from every age and every generation and was, therefore, not found in the scriptures of the prophets. In essence, It is the final revealing of the Mystery of Christ. The Mystery in Ro 16 is totally different than the new Mystery found in Paul's last 7 epistles
One is the mystery of Christ and the other the mystery of the gospel. They are not the same mystery. The mystery of Christ/ that “Christ died for our sins according to the scriptures" was a mystery, but "according to the scriptures". It's called the hidden wisdom of God in a mystery which God ordained before the world unto "our" glory (a further study of the "our" would be those to whom Paul preached 1 Corinthians 15:1-4 KJV to during Acts (Acts 13:16 KJV, Acts 13:26 KJV, Acts 26:17 KJV). See that it was a mystery here and why God kept it secret here (Romans 16:25-27 KJV, 1 Corinthians 2:6-8 KJV).
“The mystery of the gospel” is a later revealed mystery that included the likes of us, like the Ephesians to whom Paul wrote the letter, who in time past were without hope and without God in the world (Ephesians 2:11-12 KJV) but now were made fellowheirs and of the same Body and partakers of God’s promise in Christ by the gospel with Paul, the Romans, Corinthians, Galatians, Thessalonians…(Ephesians 3:6 KJV). This mystery was testified at a later due time (2 Corinthians 5:19 KJV, Acts 20:24 KJV, Ephesians 2:13-18 KJV, 1 Timothy 2:4-6 KJV, Titus 2:11 KJV)!
Together all of those saved from the beginning of Paul's ministry, the "we" and the "ye" (Ephesians 1:12-13 KJV) make up the one new man! They are all in the same one Body that we are as fellowheirs with them (Ephesians 3:6 KJV, 1 Corinthians 12:13 KJV)! It is the fellowship of the mystery which from the beginning of the world hath been hid in God,...:
Notice: "hid in God" not a mystery “according to the scriptures”.
Ephesians 3:9 And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ:
Ephesians 3:10 To the intent that now unto the principalities and powers in heavenly places might be known by the church the manifold wisdom of God,
Ephesians 3:11 According to the eternal purpose which he purposed in Christ Jesus our Lord:
Ephesians 3:12 In whom we have boldness and access with confidence by the faith of him.