Thanks for the answer.
I would not expect such a limitation between network and external usb drive attached to router. I thought the speed would be quite similiar as attaching the disk to usb 3.0 slot on laptop and copying the files onto it.
One thing that I also did before doing those changes that you have recommended me to do was, that I did a factory reset of router.
About 2.4 ghz question, I was thinking about turning 2.4ghz wifi access and leave only 5ghz wifi network available for devices to connect to it, but doing a quick look over the router settings haven't found that option...
Now I'm dealing with a new challenge but don't know if this is the right forum to ask or should i start a new thread.
I just found out two days ago that this asus router supports downloading torrents with DOWNLOAD MASTER app. I had a sparkle in my eyes for a moment, but my first experience was very crappy. The speed is not what I would expected. I have 300 Mbit download and 50 Mbit upload contract with my ISP an speedtest gives me that kind of results, but download master's max download speed gets to the 100 Mbit and yes for the test purpouse I took a torrent with a lot of active seeders.
Also after couple of hours download master just stopps working and I need to reboot the router. Very unstable app.
The app is on latest version and I also have router on latest Merlin firmware...
This are currently two bad experiences with router which I never would expect to happen. I thought this router is more 'professional' as it doesnt cost 50 dollars/euros.