I have 2 LTM bigip running 184.108.40.206 in a HA pair. When i update the passive member to 220.127.116.11 it comes back up as 'offline' and 'disconnected'. The unchanged primary is online (active) + disconnected.
I dont recall seeing this behaviour when i have updated in the past. My recollection is that they come up as standby (not offline) and complain of software version mismatch. Is there something i can check to verify that the updated member is offline because of the software mismatch and not because something is broken?
Maybe you've used 'Force offline' instead of 'Force to Standby' and the upgraded member came back 'Offline' and 'Disconnected'. The 'Disconnected' part is okay, because it can't sync between two members not running the same software version.
I would just check if a 'tmsh load sys config verify' looks okay and check the results of 'tmsh show sys mcp-state', on the upgraded member and bring it online again.