@AaronPratt 

Since the internet connection was stable for weeks, and it suddenly disconnects from the WAN, we will suggest you contacting the ISP to check if there is something wrong with the line or the modem. You can also connect a PC directly to the ISP modem, then check if you can get a stable connection.

If the ISP modem is stable, then try to do a MAC clone on the AX6000, by cloning the wired PC's MAC to the AX6000, then check if the connection between the router and the modem is stable:
https://www.tp-link.com/support/faq/3023/

BTW, who is your ISP? Is the AX6000 WAN connection type PPPoE or Dynamic IP? Please also verify if there is still a WAN IP address on the AX6000 when the internet LED becomes red. If there is still a WAN IP address, please try to change the internet DNS server to something public, such as 8.8.8.8 and 8.8.4.4, then save and reboot the AX6000 and modem.

As for the speed issue, is the 400 down or 20 down tested on a wired PC or on a laptop or phone? What do you mean by reset? Do you mean power off the AX6000 then power back on? Please refer to this article to check the QoS settings on the AX6000 again:

https://community.tp-link.com/en/home/forum/topic/257504