It is very hard for an individual (or a group) to debug your whole program (its design and its implementation) via a forum. This is not likely to succeed, in my opinion.
To have success in a Q&A forum, the problem statement needs to be minimized.
In your case, @fari35 , I would highly recommend that you start by only compiling and running the exact "untouched" sample code first:
https://github.com/qt/qtbase/tree/fc9cda5f08ac848e88f63dd4a07c08b2fbc6bf17/examples/network/threadedfortuneserver
Also compiling and run the exact unedited official client sample:
https://github.com/qt/qtbase/tree/fc9cda5f08ac848e88f63dd4a07c08b2fbc6bf17/examples/network/fortuneclient
Make sure all of that works first. I was able to compile and launch both of those with no issue, and this is what I see:
Screenshot from 2020-12-21 09-30-19.png
Once you do that, then start tracking your changes using git, and make one tiny change a time (in order to work slowly towards whatever your goal or assignment is).
When you make one tiny change (as small as editing or adding just one line of code) and the change fails to do what you expect, then you can use the time-tested bug-reporting technique of:
State what you did (the single tiny change).
State what you expected.
State what actually happened.