From my experience the answer would be yes. I have not tested this with a 67C with cellular only though. 1- You can use a cellular modem at the very least to be a primary uplink, but it won't be marked as primary. It should actually work with the sim in the device so you shouldn't need the usb modem, but that would be worst case. 2- SD-WAN has worked for me when using a usb modem, so I would say yes it will work with the internal sim Keep in mind my answers with this are with running for less than 30 days in these types of configurations, so I may not be aware of using this method long term. I have tested on a 64/65W/67 models with a cellular modem only. I don't believe you can mark cellular as primary, but while it is in use SD-WAN works.
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