@SylvesterChua 

Hi, Have you tried to keep the Unifi router as the main DHCP server, and then set Deco M4 into Access Point mode?

Are you able to test the Ethernet connection directly from the main Deco M4?

Please also try to temporarily turn off 2.4 GHz band under the main network, then run "Network Optimization" once under Deco APP>More to see whether it could improve the 5 GHz wireless speed a little bit.

Thank you very much and best regards. 

  @SylvesterChua 

Hi, Thanks for the update.

"I used Deco M4 as router (Unifi) Login. The speed is not improving."

Do you mean the internet speed of M4 is still capped at 250m/s under Access Point mode? Or you haven't tried Access Point, Deco M4 still works in router mode?

If possible, I hope you could help me test the network speed on M4 when it is in AP mode since I have seen other users mentioned once before the PPPOE connection on Deco was not as fast as Dynamic IP. I wonder whether that might be your case.

Moreover, please also help me check the 5GHz wireless link speed on the device when you are testing the speed:

How to check the associated/link speed on a wireless client

Then make sure the QOS has been disabled on the Deco APP.

Best regards.