After few days of troubleshooting and researching and reading this forum as well, it seems my c2300 v1 (EU version) has the infamous QoS bug where I can't fully disable the QoS (even when on standard settings) and it randomly decides to cut my gigabit speed in half. Only restart fixes the router and the speed, I tried playing with all settings, wasted few days of my time, nothing works. 
So it seems Tp-link doesn't want to solve this issue and just let its users to suffer or buy a new router. On top of it I don't have the option of flashing with openwrt or some other open source firmware because of the broadcom cpu. 

I'm kindly asking the support here, is it possible to provide me with some link for C2300 v1 (EU) very first firmware, the one it was released it? It seems it didn't have QoS back then, the QoS was added with firmware 2.0.1 Build 20171121 Rel. 61622 . I tried 4 different firmwares, same issue persists, but I can't find the firmware that my router shipped with, the very very first one. Please guys, if you have any chance of locating this I would be very grateful to try it, since it will solve the QoS issue. It's probably just the version before
/2.0.1 Build 20171121 Rel. 61622/

Thank you in advance.