@NX3 

What is the model number of the Deco unit? Is it Deco M9 plus or Deco P9? 

Please tell us the current network diagram, like how do they connect to each other.

Have you ever tried to forget the network/remove the profile and reconnect to the wifi network? Instruction as below: 

https://www.tp-link.com/support/faq/918/ (take windows 10 as example)

At that time, can you get internet from the Deco by wired cable？

Try to disable fast roaming and setup QoS as standard and clear all high priorit. Open Deco app, go to more-Qos to do that.

May it help and have a good day.