In the event the federal government dissolved, what would happen to the Native American reservations

[Libertas Vel Mors]

Tribal sovereignty predates the United States and would persist after it.

Tribal sovereignty is a myth that is theoretically enforced by the federal government holding "Indian land" in trust and thus stopping state sovereignty from going into effect. If the federal government ceased to exist, "Indian land" would be held by the states under the same provisions as any other state land, and the reservations would disappear.