Hi @paulreveco you mentioned MX64C but did you mean MX67C? Also, is this by any chance an active/standby pair of MX64 (or MX67C) appliances, and perhaps the primary MX LED is solid white but the standby MX is solid violet/purple? I have heard of that type of issue before with an active/standby failover pair, and Meraki Support can correct that known issue via a support case.
However that issue I've heard of is just a cosmetic issue, while in your case it sounds like the MX is falling offline, even though you have proven the SIM card works fine by swapping it into a call phone. Was the MX factory-reset at some point for any reason? Because you will need to have wired Internet connectivity for initial setup (and cellular config) prior to having ONLY cellular as your WAN uplink.
So if you do have an alternate means of (perhaps temporarily) getting this MX a wired (non-cellular) Internet connection on its wired Internet port, then you might try paper-clipping (factory resetting) the MX and allow it to come up and grab its configuration. I would suggest opening a Meraki Support ticket to walk through the process.