@yossi99 

Hi, sorry for the delay.

 Could you please help me check the following information:

1. How are you 4 Decos connected to each other, wirelessly or via Ethernet cables?

2. If only main Deco left, how much speed you could get from both wireless connection and Ethernet connection?

3. How long have you had the Deco and when did this issue start?

4. please make sure the QoS and parental control, antivirus has been disabled already;

If possible, have you considered changing another Deco as the main unit to have a look:

https://www.tp-link.com/support/faq/2400/

Thank you very much.

and Nice to see you, @RaviS! I have reply to you on another post and please have a look.