You need to contact support, rolling back firmware is fine short term but does mean you are without any of the security fixes from FW17 and at some point that firmware will not be supported.
I have found USB modems for all vendors over the years have been a pain not just Meraki, the problem is there are so many options that where do you draw the line on what you can support.
Your customers given the MX64 is old might be better using the MX68cw which has built in cellular.
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