We have the same set up across an OTV link and have tested this scenario out multiple times. You don't have to do anything if communication is broken between the prim and secondary nodes. The secondary will automatically start authenticating devices that it is in contact with. If you promote the secondary to primary after the link is broke it will assume the primary role when the link is restored and force the former primary nodes to secondary.
when primary Monitoring ISE node goes down, the secondary Monitoring ISE node automatically becomes the primary Monitoring ISE node.
You have to manually promote the secondary Administration ISE node to primary . There is no automatic failover for the Administration persona and no reboot needed for Secondary ISE after promoting admin role to primary