Let's take this slowly.
25Now to him that is of power to stablish you according to my gospel,
That is about Jesus being the him who is of power to establish you according to God's gospel.
and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began, 26But now is made manifest, and by the scriptures of the prophets,
That is the scriptures of the prophets in the Old Testament.
according to the commandment of the everlasting God,
God gave the commandment.
made known to all nations for the obedience of faith:
We must all have faith and obey, which is living faith not dead, as faith without obedience is.
27To God only wise, be glory through Jesus Christ for ever. Amen. {Written to the Romans from Corinthus, and sent by Phebe servant of the church at Cenchrea.
Take this how you will...
The structure of those ending passages in Romans 16 is that Paul is referring both to his own preaching, but also, to his (Paul's own) God commanded writings.
He also mentions that in chapter 15.
And he is basically looking back to his stated intent in Romans 1, where he first mentioned where he got his authority from, and then mentioned why he wrote to the Romans.
By the end of Romans he has both laid out the details of the gospel of Christ and also gone on from there to build on that all that material in Romans beyond the gospel of Christ - which is a part of properly understanding the mystery of Christ.
Why has he laid all that info out?
Towards their being able to be stable in the many aspects of The Faith (gospel of [good news about] Christ according to [in line with] the revelation of the mystery).
Towards their not being ignorant neither in their understanding of who they are as Body members, nor of the differences between them as members of the Body of Christ (mystery of Christ) and God's Prophesied plans and purpose for Israel.
Ephesians than builds on what he laid out in Romans.
Hence; the various summaries in Ephesians of material gone over in much more detail in Romans - and vice-versa.
Rom. 5: 6-8.