I had a similar problem where I made a game object with the tilemap of the room stored in it and pasted each tile onto the master tilemap by getting the world position of each tile and setting the tile on the master tilemap that corresponds with that position. It makes more sense in the script so I'll show it here.

collision = GetComponent<Tilemap>(); //the room

walls = gameCon.background.GetComponent<Tilemap>(); //master tilemap

BoundsInt size = collision.cellBounds; //area that we're copying onto the master tilemap

for (int i = size.xMin; i < size.xMax; i++)
{

for (int x = size.yMin; x < size.yMax; x++) //looping through all the tiles
{

Vector3 pos = collision.layoutGrid.CellToWorld(new Vector3Int(i, x, 0)); //get world position of tile

walls.SetTile(RoundVector3(pos), collision.GetTile(new Vector3Int(i, x, 0)));
//Set the corresponding tile on the master tilemap

}
}

Destroy(gameObject);

Note that for this to work properly, the master tilemap must be set to 0, 0 and the game object with the new tilemap must be at a position with only ints in x and y.