That is true, but the Orbi is a Mesh Network which blends the two together. Like @Newshound , I have had no issue. I have Mesh Network and left my phone connected as it was and simply setup my devices. As long as it was on the same network, you should have no issue. I would try this first.
If you did want to have it all on the 2.4ghz, you would go into the settings and activate your Guest Network as the other posts have indicated. However, there will be a setting where you can set it to use 2.4Ghz. But once you connect it to that Network, you will need to stick with it from that point on as it will have a separate SSID and password from your real internal network.