@David-TP  Thanks for the answer.
I actually experience huge down-and upload variations ( WiFi and Ethernet) on my DecoX75 system ( 3 units- main one set as access point behind iSP router), and thought the problem came from the firmware.
I checked the network without Deco and it works fine.
I have the EU v 3 version units which upgraded lately to 1.3.0 - 20250722. Strangely enough, the latest version on the TP Link site for Belgium is the 1.2.14 build.
All units are connected to the WiFi Backhaul

I also downloaded a log file but cannot find a way to attach it here (click on the file icon but get a message "failed to upload")

Follow up

I installed the Beta version, rebooted everything, same problem: the first few minutes download speed is OK ( in my case app 87Mb/s ( VDSL) and then it drops to about 15. This is frustrating

  @Pecoflyer 

Hello, I also live in Belgium and I also have problems with the Deco xe75 pro, I have already tried many ways to solve the problems. I also thought it was a problem with the Internet operator (formerly Orange and now I'm Proximus) and I've realized that it's not the Internet operator but the DECO. 

Could you tell us what kind of problems you have specifically? 

The problem I am having is the youtube app on Samsung devices. Specifically the Shorts and the truth is annoying. 

@david_tp 

I used a wrong DNS on my smartphone to cope with some ad issues I have with Surfshark. So, the Deco was absolutely not at fault, works great for the moment. Why the Ethernet connection was affected is a mystery. Sorry for the bother, and thanks for the help

  @Sleater14 

Sorry to hear you have problems. I don't think I can help you ( or might you also have a DNS issue ? )

  @Pecoflyer Thank you for your comment. I'm sorry to say that I've already taken the advice from tp-link technical support. They recommended that I change the DNS to Google and cloudfire and I still continue with these problems.