@oddpast 

Surfshark shows it can work with L2TP so you can try to set up the router that way to see if it works.

Here is how you would set up your VPN connection: https://www.tp-link.com/us/support/faq/1950/

If that does not work, then you may need to switch back the WAN back to what it was so it gets internet again, and then run the VPN client software on whichever device you need it on. The router has VPN passthrough so it will not block any encrypted traffic to Surfshark.

If you have the C9 router, lookup your router settings in the "VPN" section. Usually 192.168.0.1 --> login --> Advanced --> VPN

Do you see OpenVPN there? Cause OpenVPN UDP is usually what you want, most secure VPN (udp is faster than tcp). But not all variants of the C9 has it, depends on the HW version and firmware for EU/US.

If you have it, just follow the instructions: 

https://www.tp-link.com/us/support/faq/1236/

https://support.surfshark.com/hc/en-us/articles/360006143714-How-to-set-up-OpenVPN-client-on-Android-