@TP-Link 

Good morning to you!  I have the decos connected as following:

Main deco connected to FIOS access point, and then second port connected to switch.

Second deco is connected using a network backbone from the switch the main deco is connected to and second port to one device. 

Third deco is using the wireless backbone and connected to one device.

I tried your simple test in a few ways.

First disconnected it from the FIOS and tried to change it the LAN IP to 192.168.3.1, connected it back to FIOS and when the deco came back on, it was not set to 192.168.0.1.

I then went to the deco, disconnected both network cables connected to ip, set the LAP IP in the deco app to 192.168.1.3 while it was on 5G.  I then connected the main deco back to FIOS, and then connected iPhone to it.  LAP IP is 192.168.0.1.  For some odd reason i cannot get it back to 192.168.3.1.  Let me attach screen shots.  

@TP-Link    Hello,

     I had some time to try and disconnect it again, reboot the FIOS and even tried to change it to others IPs such as 192.168.3.3 and so on and it refuses to go into the 3 range.  I do not see how I can possible change the LAN IP if I have the main deco disconnected from the internet anyways.  But I have attached more images (smaller size) for review.  I was wondering if maybe I need to revert the firmware?

@TP-Link Hello!  To help you better understand , I still cannot set any LAN IP in the 192.168.3.X range. I can change the LAN IP to say the 192.168.4.X range. I do not understand how I can disconnect the main deco from the FIOS modem since the app then shows the deco is offline. I did try that and as you see in the one image, shows deco offline. I did try disconnecting the main deco from the TP-Link switch going down but still cannot set the LAN IP back to 192.168.3.X.  Any ideas?  Any further questions?