BigIP LTM Standby device is showing offline on the active device

I had a similar issue while trying to add the Standby to HA the F5 KB steps recommend putting Standby in a "Forced Offline" state while preparing sync. Check the HA heartbeat Self-IP settings and be sure the port lockdown mode is "allowed default" The default setting is "allow none" which blocks the heartbeat but CMI is the exception to "allow none" so sync works. https://support.f5.com/csp/article/K13250

Doubt this is your problem but this came up in my search when I had the issue and may help others.