Hey there! @Nick_Rodgers, thank you for providing that detailed information on the troubleshooting steps that you have already attempted.
Two suggestions that I noticed that was not mentioned; would be to attempt to clear the storage cache of your ST Application on your Iphones, and to uninstall and reinstall the ST Application to ensure your running the latest versions, and see if you are able to onboard the device or received the error code again.
After reviewing the User manual in regards to the onboarding process, Did you attempt to add the device by QR Code or Serial Number located underneath the QR code?
I was also able to review your model ‘DW60A8060BB’ and it is an international UK model, There are differences between the US and International firmware, which I do not have access to. As your product is a non-US model and support for these models is very limited, please seek a support team for your area if the symptom persists.
You can reach out directly to a support team in your area using the below information:
UK: 0333 0000333
I hope this message finds you well!