Slow internet speeds after a while

Hello, I am having problems with my internet speeds.
I live in Romania and my internet provider is DIGI, my internet speeds in my region are 900+ download and 450+ upload constantly.
On my router I am getting 300 download and about 150 upload, it doesn't exceed 400mb/s.
I contacted my ISP, they resetted the modem a few times, everything checked out, after that I factory resetted my router...internet speeds back in business(900down,450up) and after 1 week or a few days it goes back to 300 again...I am having this issue for some time now.
I tried changing the firmware on the router to an older one, no succes.
Tried different settings, no succes.
Restarting router+modem, no succes.
Every time is the same thing and I need to reset my router completely.
QOS is always off, firmware version is the latest(V1.0.3.64_1.0.1), hardware verison(R1.05).
My settings are default, I configured it with the netgear network app(nighthawk).
If I can provide any more information, feel free to ask.
Thank you !