I had this problem and solved it. I had a 'Coles' Optus sim which worked fine in a 2021 Range Rover Sport. However, a data sim (additional to my post-paid plan) from Optus would not connect.
After going through many rounds with Optus support (very painful) I eventually chatted to their social media support team who suggested changing the APN. You can do this through the Range Rover by selection 'manual':
Network provider: The network provider of the inserted SIM is displayed. To change the APN settings, select the network provider's name to open the NETWORK PROVIDER screen. Set Auto select to Off. Add the Network, Username, and Password. Select Connect.
For my plan/type, I entered 'connectme'. No username or password required.
APN can be found here -> APNs & technical settings for Mobile Broadband & Home Wireless Broadband (optus.com.au)
Range Rover Sport manual -> Land Rover Owner Information