If you believe the documentation on NordVPN's website, TP-Link routers are incompatible with thier service. However, I was able to do this in April 2022 with an Archer AX20 router, and found it quite straight forward to set up.

Steps below:

1.   Navigate to (nordvpn dot com)/servers/tools/

2.   Find and select the server you'd like to connect through and expand the "Show available protocols" section

3.   Download the OpenVPN TCP file

4.   Navigate to (my dot nordaccount dot com)/dashboard/nordvpn

5.   Find and copy your username and password from the Service Credentials section

6.   Log into your router by navigating to it's IP address, or via http://tplinkwifi.net/

7.   Navigate to Advanced>VPN Client

8.   Enable VPN and Save

9.   Under "Server List", press ”Add"

10. Enter a name under "Description" (I used the name of the country I was connecting through)

11. Under "VPN Type", select OpenVPN

12. Paste the username and password you copied earlier into the appropriate fields

13. Upload the config file you downloaded earlier

14. Save

15. Repeat the previous steps for any additional servers you'd like to be able to connect through

16. Under "Device List", select all devices that you would like to connect through the VPN, when enabled

17. Under "Server List", use the "Enable" toggles to connect and disconnect from specific VPNs

Hope that's useful to someone else.