ISPs in Turkey will tell you a lie along the lines of "You will get 20% lower speeds on 3rd party routers" because they don't want to bother debugging issues on routers they don't control. I know that's not true since I get all the speed I pay for, but to connect at all I have to clone the ISP provided router's mac address because 3rd party routers aren't supported.
It's not impossible that your ISP could be doing something similar where they say they support 3rd party routers but throttle speeds based on your router's mac. If you do end up asking for an ISP router and get better speeds on it, you could try cloning the mac and see if that solves your issue for your own router.