See also the Amish.
https://www.bloomberg.com/news/articles/2012-08-01/why-the-a...
https://www.bloomberg.com/news/articles/2012-08-01/why-the-a...
Native American fertility is actually below replacement (and lower than white, last I saw). But the definition of NA is more expansive, and the population can "expand" through outmarriage.
You see something very similar going on with the Reform Jewish population. Reform Jews have relatively few kids, but a very high intermarriage rate (about 80% of Reform Jews marry non-Jews!) so you see the Reform Jewish population increasing regardless.
Both the Amish and the Haredim reject a lot of modernity and are quite conservative as well look to keep traditional families and gender roles. Probably not a coincidence that there are the groups still seeing strong growth.