configsync issue "No CMI peer devices configured"

Hi guys, have a problem with configsync.

I’ve established trust between a pair of F5 running 11.5.3 using the management interface. Configsync is set to use an HA vlan running over dedicated 2x10GE trunk group. The self IP is set to allow ‘default’ I’ve configured on both devices configsync to use the HA vlan. I can ping the other device HA interface. Each devices shows the other as offline, if I sync the primary with the other in the log I see:

Sep 13 17:16:18 slb-2a notice mcpd[7286]: 01071436:5: CMI listener established at 10.120.255.65 port 6699 Sep 13 17:16:18 slb-2a notice mcpd[7286]: 01071434:5: No CMI peer devices configured

On both devices port 6699 (TCP?) is not listening:

netstat -pan | grep -E "1026|6699"
udp        0      0 10.120.255.66:1026          0.0.0.0:*                               7462/sod

cm device-group cluster-failover {
    auto-sync enabled
    devices {
        slb-2a.local { }
        slb-2b.local { }
    }
    type sync-failover
}
cm device-group device_trust_group {
    auto-sync enabled
    devices {
        slb-2a.local { }
        slb-2b.local { }
    }
    network-failover disabled
}