@Morpheus83 

Do you mean you can get a normal download and upload speed when you connect the computer to the Archer C7 with an Ethernet cable? Please check this first.

Follow the guide below to check and confirm if the QoS settings on the C7 is configured correctly, you can try to disable it if necessary:
https://www.tp-link.com/support/faq/3069/

Just to confirm, what kind of wireless devices you are testing the 5G connection with the C7, can we have the brand and model of it? If it is a computer, we would like to know the model of it, please ensure it is running with the latest driver, thanks.

You can also check and modify the 5G wireless settings on the C7, you can check that by installing an app like Wi-Fi Analyzer on your phone or laptop and test, then set a channel with less congestion according to the test results on the C7, and change the channel width for the 5GHz to 80MHz, reboot the C7 and reconnect to the network then conform.