Hi,

Without details it's very difficult to help you, I mean from and to where are you trying to download? what is the client wifi adapter used?

I have the same problem. Max download speed of around 50.000 kbit/s in 2.4 GHz but 100.000 kbit/s in 5 GHz. I also have the Archer C7

i just used speedtest.net
and if i try to download a torrent my max speed is 5,5 mb/s
i have a 300 mb band from my provider

Have you tried to make the 2.4GHz speedtests on some other devices such as smartphones?
As for the device with very low 2.4GHz speed, please check its wireless link speed (open network and sharing center, click the wireless connection and check the speed in Details).
In addition, please also try to fix the 2.4GHz wireless bandwidth to 40MHz (login the router's management page, refer to Wireless -> Wireless Settings, and fix the bandwidth).

i tried on smartphone: 2.4 ghz is also 50 mb/s
5 ghz band is 160+ mb/s

i checked the wireless link speed how you sugested it's 72,2 mb/s

the router is curently working on both 20 and 40 mhz

same problem here using archer c2 750