That link looks like a setup for the OpenVPN Server on your router. You need the do a setup for OpenVPN Client. If you do a search on google for asus gt-ac5300 Openvpn Client setup you might find some info. If PIA does not have a setup for this router, you might be able to use the instructions for another router, The information is usually about the same.
Comments
https://www.asus.com/support/FAQ/1008713/
Use the Openvpn setup not PPTP