Your iPhone should automatically shift to another cellular signal, such as 4G or LTE, when out of range of a 5G tower. You can’t do anything in this case. If you are having issues with your 5G connection, like it’s not installing or uploading data, you can manually switch to an alternative option. Go to settings > Mobile > Mobile Data options > Voice & Data and select your preferred connection. Most providers no longer offer you the option to select a slower connection. These signals typically automatically back up if the 4G network fails.