Thanks @Clive_A  for your help. 

Please find the network diagram attached. 

you mentioned, 

"Note that the adoption loop is usually a problem with the network configuration." . 

could you please elaborate on what would this misconfiguration be? 

I also went through your link to troubleshoot the adoption issues. However, the only scenario applicable to us is 

Scenario 4: Configuring -> Disconnected loop. 

the troubleshooting steps asks to check if "Wired Network" (Settings -> Network -> Internet) is configured properly. I double checked, and it is configured properly. Please note this was all working perfectly fine until yesterday.  So we are really surprise as to why it would fail suddenly. We have even successfully restarted whole system many times in the past. If it was mis-configured from our end, it would have failed earlier as well. 

If we disconnect the router from controller and configure in standalone mode with exact same settings on controller. it all works. It's just when we adopt it on controller. Adoption happens fine, but it is stuck in "Configuring <--> Disconnected" loop. 

It was running perfectly fine for last 4-5 months. We can't figure why this happened all of a sudden. Little bit concerned on the reliability. 

Thanks again for your quick reply. We really appreciate your help.  

also attached is the configuration export from controller for support.  If it would help in further diagnosis