I have the same robot cleaner. It's working fine here, but I think it's because it joined my wifi network when I had an older system - the network name and password stayed the same. I think because it already knew the details it just connected and off it went.

So perhaps borrow an older wifi router or even an simple access point and set it up with the same authentication details as your new network (you'll need to make sure you use the same authentication type like WPA2). Then turn off deco and set the robot up. Then switch the temporary one off and turn Deco on again. The robot should be able to connect.

You could probably even rename your phone to the Deco's wifi name and turn on "personal hotspot" feature?

So i have a workabout for this with suggest from TP-Link's support.

I turned off all m5s except for the main one that's connected to my modem.
Off i go to the furthest corner of my home, and my phone will connect to the 2.4ghz (given the characteristics of this frequency), set the robo vacuum to search mode, and voila, the robo vacumm connects to the 2.4ghz now.
After setting up, I switched on the rest of the m5s and have no issues thereafter.

Suggested to TP-link support to have an option of turning off the m5's 5ghz option in the app as there are more smart devices riding on the 2.4ghz frequency. They will bring this up and we will see if there's any changes.