@Linson 

Have you tried the following to reset the unit or is it not working? 

To factory reset a TP-Link Deco router, locate the small reset button on the unit's bottom or back, then press and hold it with a paper clip for approximately 5-10 seconds until the LED light begins to flash or change color, such as blinking yellow or orange, indicating the reset is in progress. After the reset completes, the unit will restart, and you'll need to set it up again from scratch using the TP-Link Deco app

  @Linson 

Hi, the hardware reset of Deco only needs to hold the reset pinhole for 1s.

If it didn't respond to a hardware reset, here are some suggestions for your reference:

• Swap the power adapter of this Deco M4 with another M4 unit.
• Refer to the FAQ to see whether it could get into firmware recovery more:How to use firmware recovery if the Deco become brick >HTTP Uboot Tool

By the way, may I know how long you have had the Deco?

Wait for your reply.

Best regards.