@TandS 

Did the firmware release for the APs that enabled the "lock to ap" setting?

Not sure how I'm supposed to know that. I'm running the latest firmware version available and the Omada software allows the setting without any errors or warnings so I have no reason to believe that the feature wouldn't work.

That "might" explain the Lock to AP function but if you look at my screenshot it only mentions that the device "after enabling lockto AP, clients may ..... ".  No mention about your firmware not being supported like you "now" see elsewhere in the SDN. 

Here is an example  showing a feature not currently supported which is what I would expect to see if it might not work:

And what about the Fixed IP not working again. This issue was supposedly fixed months ago but appears to have resurfaced again. Internal Quality Control on these products appear to atrocious.  I can now understand why so many people just go with Unifi products.  This might be cheaper but I guess you ultimately get what you pay for.  If I was having to rely on this for a  business it would have been ripped out and replaced months ago now.

  @TandS 

Did the fixed IP issue occur when you reboot the controller?

Have you tried other IPs beyond the DHCP pool?

Maybe rebooting the gateway can fix it.

  @Virgo 

Upon closer inspection I did observe that another client device was using the IP address that was "supposed" to be fixed to the client in question.

The client that was using this IP address did not have a Fixed IP address associated it to and the client with the problematic Fixed IP address had this address reservation for many weeks now.  I can only guess that one of the recent updates either to the SDN software or to the ER605 brought the issue back.

To try and resolve my issue I changed the Fixed IP address to the one it was "currently" assigned and then I went into my home automation applications and revised the IP address to reflect the changed one.

Bottom line is this should not be happening and I hope TP-Link takes this seriously and investigates as this has been an ongoing issue for many for well over a year now and should be considered basic functionality in a router.

  @TandS 

Wait... so is the fix IP issues... assigned a fixed IP that is WITHIN the DHCP Scope or outside the scope?

It works for me within the DHCP scope.  

Why would you enter a fixed IP (DHCP Reserve) outside the DCHP scope?  Just manually statically assign it.

It was assigned a fixed ip address within the scope of the DHCP range.

I have since just lowered the DHCP range and manually assigned an IP address outside the range in the hope this doesn't occur again.