@garian 

Hello, please help provide below details, and we will try to help you out, thanks:

1. What is the download/upload speed you can get when you connect a computer directly to the ISP modem?

2. Without any other devices connected, connect one computer to the C9 directly and go to speedtest to run the test for 3 times and show us the screenshots of the test results. BTW, simplify your network layout, just leave the modem and the C9 on, disconnect the RE305 and run the speedtest.
And please tell me the wired link speed at that time, how to get that? You can refer to the below link.
https://www.tp-link.com/en/faq-2265.html

3. Do you enable any QoS setting on the C9? Disable the QoS setting, then confirm again. Or you are suggested to reset the router to factory defaults and reconfigure it from scratch, then confirm.

4. Try a mobile app to analyze the Wi-Fi around you, review all the wireless channels. Next, review the wireless channels on the router. If the channels match that around you, an indication of overcrowded. Manually adjust the channels to lesser use channels.
There are possibly other wireless parameters that need tweaking, such as Power Transmit, narrow or wide band, etc. but start with the above and below link:
https://community.tp-link.com/en/home/forum/topic/200588​​​​