Hey, BDub,
Hugely useful information – thank you!
CGDCONT status below.
YES! There was indeed information written to the Forbidden Zone of Forbidden Providers. Cleared.
Connected just fine today, then checked:
AT+CGDCONT?
+CGDCONT: 1,“IP”,“hologram”,“10.155.172.215”,0,0,0,0
Check the Blacklist:
AT+CRSM=176,28539,0,0,12
+CRSM: 144,0,“130184FFFFFFFFFFFFFFFFFF”
Clear the Blacklist:
at+crsm=214,28539,0,0,12,“FFFFFFFFFFFFFFFFFFFFFFFF”
+CRSM: 144,0,""
Verify empty Blacklist:
AT+CRSM=176,28539,0,0,12
+CRSM: 144,0,“FFFFFFFFFFFFFFFFFFFFFFFF”
Definitely going to be reading up on SIM technology / settings. Thanks for pointing me in the right direction.
Also, put me down on this wishlist: “AT command passthrough mode” app for Electron and Boron. Really makes this type of work easy and fast after the Arduino “SARA pass-through” sketch is loaded. especially for those of us who grew up on “speaking Hayes” since way too long ago. 
UPDATE: Mar 7, AM: All SIM’s working well today. Lesson learned: Don’t willy-nilly switch SIM’s among devices; lest the Cellular Tower Gods grow angry.