create anchor arcore
if (this.anchorNode == null) {
Session session = arFragment.getArSceneView().getSession();
float[] position = { 0, 0, -0.75 }; // 75 cm away from camera
float[] rotation = { 0, 0, 0, 1 };
Anchor anchor = session.createAnchor(new Pose(position, rotation));
anchorNode = new AnchorNode(anchor);
anchorNode.setRenderable(yourModelRenderable);
anchorNode.setParent(arFragment.getArSceneView().getScene());
}