LTM Peers disconnected after Blade replacement

Delete the device from the peer list and/or subordinate list, in both devices. In each device, go to device groups, select the current device and make sure all the device connectivity is setup (config sync, network failover, mirroring). Try add again the the peer device in the peer list. After check in each device, the device list, and check if both devices have now all information.

Don't forget that are multiple requirements for the device be able to do HA stuff. See this solution for more details:

https://support.f5.com/csp/article/K8665