How could former pagan Christians in the 12 countries and 63 cities mentioned in the N.T. know what to do/not do if they did not have anything to tell them the laws of God?
There is no record in the N.T. that believers in those 12 countries and 63 cities were ever provided any Old Testament commandments. How could they follow the O.T. when they did not have it.
The Ten Commandments were repeated in the New Covenant, and it was a teaching of Jesus and the apostles. Paul said what matters is keeping the commandments of God 1 Cor 7:19 and pointed right to the Ten Commandments to define sin. Romans 7:7
The Law of God in the New Testament.
1. "You shall worship the Lord your God, and Him only you shall serve" (Matthew 4:10).
2. "Little children, keep yourselves from idols" (1 John 5:21). "Since we are the offspring of God, we ought not to think that the Divine Nature is like gold or silver or stone, something shaped by art and man’s devising" (Acts 17:29).
3. "That the name of God and His doctrine may not be blasphemed" (1 Timothy 6:1).
4. "He has spoken in a certain place of the seventh day in this way: 'And God rested on the seventh day from all His works.' There remains therefore a rest ["keeping of a sabbath," margin] for the people of God. For he who has entered His rest has himself also ceased from his works as God did from His" (Hebrews 4:4, 9, 10).
5. "Honor your father and your mother" (Matthew 19:19).
6. "You shall not murder" (Romans 13:9).
7. "You shall not commit adultery" (Matthew 19:18).
8. "You shall not steal" (Romans 13:9).
9. "You shall not bear false witness" (Romans 13:9).
10. "You shall not covet" (Romans 7:7).
The Law of God in the Old Testament.
1. "You shall have no other gods before Me" (Exodus 20:3).
2. "You shall not make for yourself a carved image—any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth; you shall not bow down to them nor serve them. For I, the Lord your God, am a jealous God, visiting the iniquity of the fathers upon the children to the third and fourth generations of those who hate Me, but showing mercy to thousands, to those who love Me and keep My commandments" (Exodus 20:4–6).
3. "You shall not take the name of the Lord your God in vain, for the Lord will not hold him guiltless who takes His name in vain" (Exodus 20:7).
4. "Remember the Sabbath day, to keep it holy. Six days you shall labor and do all your work, but the seventh day is the Sabbath of the Lord your God. In it you shall do no work: you, nor your son, nor your daughter, nor your male servant, nor your female servant, nor your cattle, nor your stranger who is within your gates. For in six days the Lord made the heavens and the earth, the sea, and all that is in them, and rested the seventh day. Therefore the Lord blessed the Sabbath day and hallowed it" (Exodus 20:8–11).
5. "Honor your father and your mother, that your days may be long upon the land which the Lord your God is giving you" (Exodus 20:12).
6. "You shall not murder" (Exodus 20:13).
7. "You shall not commit adultery" (Exodus 20:14).
8. "You shall not steal" (Exodus 20:15).
9. "You shall not bear false witness against your neighbor" (Exodus 20:16).
10. "You shall not covet your neighbor’s house; you shall not covet your neighbor's wife, nor his male servant, nor his female servant, nor his ox, nor his donkey, nor anything that is your neighbor's" (Exodus 20:17).