For Verizon specific, here were our steps to get it to work and have VZW recognize the IMEI:
1. Put SIM into a Verizon approved device (for our client, they had a RedLion RX50 as a spare)
2. Wait for service to work, check your VZW account to ensure the IMEI is showing associated to the device
3. Connect to MG21(e) and configure the appropriate APN
4. Put SIM into MG21(e) and power up
5. Walk away, go get coffee. Upon return, if white light not blinking, power cycle MG
6. Walk away again, refill coffee. Upon return, if white light not blinking, power cycle MG
7. Repeat steps 5 & 6 a few more times
8. Verbalize the following incantation: "D*&!@t ... Just work!"
9. Your white light should now be blinking with Verizon and will ultimately turn to purple (while I like purple, bad color choice that doesn't fit the norm in my opinion)
9a. If you're a consultant like me, about the time you verbalize your client will walk in and ask if the incantation worked.
10. Deploy to your environment and test to your heart's content.
(For any Meraki mods reading why purple is bad. Short version: A client broke their ISP modem. The stock indicator colors on the device are: Blue - good, Red - bad, Orange - connecting. The onsite user decided that IT was overrated and attempted to resolve a (Meraki) firmware update issue on their own instead of waiting for our phone call. They pressed the (disabled) WPS button so hard on the ISP modem they broke the WPS daughterboard which lit two of the indicator lights. Any guesses which two? hint: go back to grade school art class - which two colors make purple? ... Anyway, I wish the MG indicator light was white/blue like everything else so we don't have to retrain users)