The door he is speaking of, daqq, is not some door of perception.
Rather, an opportunity that had been opened up by one of Paul's various co-laborers' preaching, prior to Paul's following afterwards with a visit unto them to help them in their work in the furtherance of the gospel in regions beyond where Christ had thus far been named.
Their work in advance of Paul's later follow up in person had been a recurrent pattern of his, just as had been his recurrent pattern to revisit those assemblies he himself had established.
He relates that to the Corinthians, for example - how that on his way to one of these doors of opportunities to preach and teach that have been opened at Troas by one of his, he plans to stop at Corinth to visit them.
Paul had various co-laborers helping him with various aspects of his ministry.
1 Corinthians 16:3 And when I come, whomsoever ye shall approve by your letters, them will I send to bring your liberality unto Jerusalem. 16:4 And if it be meet that I go also, they shall go with me.
16:5 Now I will come unto you, when I shall pass through Macedonia: for I do pass through Macedonia. 16:6 And it may be that I will abide, yea, and winter with you, that ye may bring me on my journey whithersoever I go. 16:7 For I will not see you now by the way; but I trust to tarry a while with you, if the Lord permit. 16:8 But I will tarry at Ephesus until Pentecost. 16:9 For a great door and effectual is opened unto me, and there are many adversaries.
There, he relates both the open door of opportunity to preach at Jerusalem that Pentecost presents, but also; the fact of his enemies at Jerusalem.
Acts 18:21 But bade them farewell, saying, I must by all means keep this feast that cometh in Jerusalem: but I will return again unto you, if God will. And he sailed from Ephesus.
Acts 19:21 After these things were ended, Paul purposed in the spirit, when he had passed through Macedonia and Achaia, to go to Jerusalem, saying, After I have been there, I must also see Rome.
Acts 20:16 For Paul had determined to sail by Ephesus, because he would not spend the time in Asia: for he hasted, if it were possible for him, to be at Jerusalem the day of Pentecost.
You'll note he'd also planned to visit those Body members at Rome.
But as he related "there are many adversaries."
Hence...
Romans 1:13 Now I would not have you ignorant, brethren, that oftentimes I purposed to come unto you, (but was let hitherto,) that I might have some fruit among you also, even as among other Gentiles.
Romans 15:18 For I will not dare to speak of any of those things which Christ hath not wrought by me, to make the Gentiles obedient, by word and deed, 15:19 Through mighty signs and wonders, by the power of the Spirit of God; so that from Jerusalem, and round about unto Illyricum, I have fully preached the gospel of Christ. 15:20 Yea, so have I strived to preach the gospel, not where Christ was named, lest I should build upon another man's foundation: 15:21 But as it is written, To whom he was not spoken of, they shall see: and they that have not heard shall understand. 15:22 For which cause also I have been much hindered from coming to you. 15:23 But now having no more place in these parts, and having a great desire these many years to come unto you; 15:24 Whensoever I take my journey into Spain, I will come to you: for I trust to see you in my journey, and to be brought on my way thitherward by you, if first I be somewhat filled with your company. 15:25 But now I go unto Jerusalem to minister unto the saints. 15:26 For it hath pleased them of Macedonia and Achaia to make a certain contribution for the poor saints which are at Jerusalem. 15:27 It hath pleased them verily; and their debtors they are. For if the Gentiles have been made partakers of their spiritual things, their duty is also to minister unto them in carnal things. 15:28 When therefore I have performed this, and have sealed to them this fruit, I will come by you into Spain.
Not only was the guy one busy individual - despite great opposition - but was ever one heck of an organizer of his time; his exit strategy; his next steps; and so on.
So much so that the places he was visiting were either first visited by him with his gospel of Christ; or already set in motion by one his various co-laborers.
Note his recurrent pattern - get a work started, grow them in an understanding of various issues in their mutual faith, then encourage them to do likewise with others.
Though his great disappointment at the Corinthians elsewhere in his writing to them was obvious, he still wrote the following unto them...
2 Corinthians 10:13 But we will not boast of things without our measure, but according to the measure of the rule which God hath distributed to us, a measure to reach even unto you. 10:14 For we stretch not ourselves beyond our measure, as though we reached not unto you: for we are come as far as to you also in preaching the gospel of Christ: 10:15 Not boasting of things without our measure, that is, of other men's labours; but having hope, when your faith is increased, that we shall be enlarged by you according to our rule abundantly, 10:16 To preach the gospel in the regions beyond you, and not to boast in another man's line of things made ready to our hand.
Note this again, as he moves on once more...
Romans 15:28 When therefore I have performed this, and have sealed to them this fruit, I will come by you into Spain. 15:29 And I am sure that, when I come unto you, I shall come in the fulness of the blessing of the gospel of Christ.
He desires to go to Rome to joy with them in their understanding of the gospel of Christ.
He has them on his mind, and covets their prayers.
15:30 Now I beseech you, brethren, for the Lord Jesus Christ's sake, and for the love of the Spirit, that ye strive together with me in your prayers to God for me; 15:31 That I may be delivered from them that do not believe in Judaea; and that my service which I have for Jerusalem may be accepted of the saints; 15:32 That I may come unto you with joy by the will of God, and may with you be refreshed. 15:33 Now the God of peace be with you all. Amen.
That all was...his recurrent pattern.
And given what Rome was back then - the very capital of one absolutely gargantuan Empire, the very hub of people from all over the world - what a door of opportunity the Romans had made use of Rome.
They too had repeated his pattern far and wide.
Romans 1:7 To all that be in Rome, beloved of God, called to be saints: Grace to you and peace from God our Father, and the Lord Jesus Christ. 1:8 First, I thank my God through Jesus Christ for you all, that your faith is spoken of throughout the whole world.
Put away Aldous Huxley ("Doors of Perception")