Alma 21:16 - 21:20
Alma 21:16 And they went forth whithersoever they were led by the Spirit of the Lord, preaching the word of God in every synagogue of the Amalekites, or in every assembly of the Lamanites where they could be admitted.
With the backing of the king in being able to preach where they wanted, the boys went from place to place preaching to whoever would listen to them.
Alma 21:17 And it came to pass that the Lord began to bless them, insomuch that they brought many to the knowledge of the truth; yea, they did convince many of their sins, and of the traditions of their fathers, which were not correct.
They began to have success among the Lamanites. The Lamanites were now being made aware of the false traditions that had been common among them in relation to the Nephites and their attitudes towards them. In addition, they were made aware of their sinful ways and shown how the gospel could help them get on the right path towards perfection.
Alma 21:18 And it came to pass that Ammon and Lamoni returned from the land of Middoni to the land of Ishmael, which was the land of their inheritance.
Ammon and Lamoni now return to the land of Ishmael now that Aaron and his friends were freed and able to carry on their ministry without fear of persecution or arest.
Alma 21:19 And king Lamoni would not suffer that Ammon should serve him, or be his servant.
And king Lamoni would not have Ammon serve him anymore.
Alma 21:20 But he caused that there should be synagogues built in the land of Ishmael; and he caused that his people, or the people who were under his reign, should assemble themselves together.
However, Lamoni had places of worship build in his land and encouraged his people to gather together to hear the word of God.