Hello neil
Thanks for responding back on this. Glad to hear that you were able to identify the issue with your PowerShell scripts.
Regarding your question about assigning your own IP addresses for an Azure Lab, it is possible to do so by configuring a custom virtual network in your lab.
You can create Virtual network with the IP address range you want to use for your Lab.
Once you have created your custom virtual network, you can assign it to your lab.
After you have assigned your custom virtual network to your lab, you can configure your VMs to use the IP addresses you specified in the virtual network.
Ref: https://learn.microsoft.com/en-us/azure/lab-services/tutorial-create-lab-with-advanced-networking
Hope this helps.