https://www.cisco.com/c/en/us/support/docs/field-notices/640/fn64006.html
Field Notice: FN - 64006 - C819HG-xx, C819HG-4G-xx, GRWIC-4G-LTE-x & CGM-3G-XXXX - Possible...
Select Connected Grid Modules (CGMs) and Integrated Services Routers that run Cisco CGOS or Cisco IOS Release 15.5(3)M1 or earlier might experience corruption...