I am hearing of a lot of folks with MX65 appliances who's USB port is not working with a SkyusDS today, anyone else experiencing this issue with any make USB modem? Have you logged a support case with Meraki?
Their is an open bug report for the MX65, please reference this when calling in to support with this issue.
On the whole I have few clients using USB for 3G/4G. I always advise them to use an Ethernet 3G/4G device where possible. Of the few using USB (maybe 200 to 250 MX devices across all my customers) - none of them are reporting any issues.