The error message 400 Bad Request, {'errors': ['A bound combined network may only contain bound networks']} suggests that the combined network configuration might be causing the problem.
Ensure that you are using the correct network IDs for the combined network. Sometimes, an incorrect network ID can cause this error.
Instead of claiming all devices at once, try claiming the MG devices separately. This might help bypass the restriction on combined networks.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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