From the screenshot of speedtest I saw that from time to time you are getting proper speeds. Have you tried to use some kind of a Wi-Fi analyzer to see if there's a big congestion of Wi-Fi APs near...

@Mr.Toni Have a look at these guides, they might be helpful: "What should I do if internet speed is slow when the device is connected to the TP-Link router?" "How to improve my wireless speed or...

If you suspect a particular device you can disconnect it during the problematic hours to see if that's the culprit. I know that not lot of people keep a spare router at home, but I was thinking that...

Can you power off the extender and the other router for a while and do the speedtest again to exclude any Wi-Fi interference or bottlenecks ?

@Psyko To use that channel you need a 3-band router ( Archer C9 is dual-band only) In the current FW there's no possibility to select a region. I would recommend these settings for the 5GHz band: The...

How far is your PC from the router ? Any walls between your PC and the router ? If you leave the channel number and channel width on Auto what's the Wi-Fi speed ? Recommended wireless settings: