Apple has introduced a new iCloud+ feature to iPhones called Private Relay, which prevents the Iridium GO app from communicating with the Iridium Go device.
If Private Relay is turned on, you will see the following error message when you try to connect to the Iridium GO.
To check this feature is turned off, go into Settings, tap on your Apple ID at the very top of the page (this usually appears as your name), tap on iCloud, and scroll down to iCloud+ features.
Make sure Private Relay is turned OFF.