Sure thing I will fill in the report now. This affected 6 different decos in exactly the same way and the only way I could regain functionality was to install firmware 1.6.4 manually back onto each one.

edit:

The box only allowed 500 chars so just to clarify even further: on any firmware past 1.6.4 if you set a connection preference on a deco the light will go red blinking and it wont communicate with either the main deco or the connection preference setting even when right beside each other. The decos seem to want to only talk to the main deco regardless of signal strength when reset and any setting other than auto renders them inoperable.
They can be reset using the button and re adopted, however again they will only talk to the main deco and if you try and set a connection preference they will lose all connection again once rebooted (they stay connected to the main deco when you set a connection preference).

The only solution was to manually roll back the firmware to 1.6.4 and everthing behaved as it should.

All decos were running the same firmware (tried multiple) when they had this bug.

  @David-TP it wasn't beta firmware it was just the standard firmware it auto updated to. 

Thatdoes definitely sound like a similar issue however in my case if I set a connection preference that wasn't already set it bricked the deco. I'm not willing to test the beta as it took over 2 hours to fix the mess after the last update and more time troubleshooting what was going on.

One deco was extremely reluctant to go into http U boot mode and took about an hour and multiple resets to finally get the web page to show.