Spanning tree

I’ve got a 24 mile shot, 5GHz and 2.4GHz bridged using STP for redundancy.

The 2.4GHz link only exists as a backup should I have a problem with the 5GHz shot.

 1  R name="L2CBridge" mtu=1500 l2mtu=2290 arp=enabled 
      mac-address=00:xxxxxxx protocol-mode=rstp priority=0x8000 
      auto-mac=yes admin-mac=00:00:00:00:00:00 max-message-age=20s 
      forward-delay=15s transmit-hold-count=6 ageing-time=5m



2    BrL2C5                 L2CBridge           0x70     10         none      
 3 I  BrL2C2                 L2CBridge           0x90     20         none

Right now it defaults to the 2.4GHz interface which is causing problems. It was never intended to be used as a backhaul, only a backup.

The other side is configured identically.

What am I doing wrong?