I was able to successfully connect my Archer AX21 WiFi 6 router to Centurylink Fiber. The process is relatively simple, and just requires using a modified Centurylink PPP and a few tweaks to the advanced settings which I found in another thread. I did contact Centurylink to find out that I needed to modify the domain of my PPP username, but this shouldn't be required for anyone else. The setup should be able to be completed using the below and no support from Centurylink (but don't quote me on that).
My successful setup process as follows:
1. Login to existing Centurylink router / modem through http://192.168.0.1/
2. Change domain of PPP username - navigate to 'Quick Setup' and change drop-down to '@qwest.net' and click 'Apply' (only change the drop-down, keep existing Centurylink generated username and password)
3. Shut down Centurylink router > connect ethernet cable (which should be coming from an already configured Centurylink modem) to TP Link router and power on
4. Login to new TP Link GUI by navigating again to http://192.168.0.1/
5. Start quick setup - select PPPoE and enter the PPP username and password from Step 2 above > follow the prompts through setup process
6. After quick setup is complete, update advanced settings (I don't remember if I did this during or after quick setup) - in the left side bar, select 'IPTV/VLAN' > check the box to enable 'IPTV/VLAN' > select 'Custom' in 'Mode' drop-down > input 'Internet VLAN ID: = 201', 'VoIP VLAN ID: 202', and 'IPTV VLAN ID: 203'
7. Enjoy your incredibly fast internet using a WiFi 6 router that you own and paid much less for
Have attached a screenshot of my 'Advanced Settings' page for reference from the TP Link GUI.