my wifi, whether using 2.4ghz or 5ghz band, the download speed is only 6mbs i'm a newbie so i don't know much hope someone can help
Download speed from where?
Using what router and client?
With the ISPs advertised internet speed of?
i download from both 2.4g and 5g bands they are under 6mbs speed with 100mbs cat 6 cable i am using xiaomi cr6608 transmitter with wifi 6 support
Answer all questions, not one.
And how is the cat6 cable related to the wifi?
The figure 6 MBit/s sounds rather suspicious, as this is the (base speed and-) value displayed by some drivers in case their firmware does its own rate control and doesn't keep mac80211 in the loop about the actual speed.
Make sure to actually measure the speed (iperf/ iperf3), not just read the figure from luci.