Network setup of 2 routers with 1 provided by ISP and 2 bought by me to host openVPN

I have ISP provided router GX Titanium-2122A (IP: 192.168.1.1), where I was able to port forward 1594 for UDP. I bought another TP link Archer C1200 router with OpenVPN server capabilities. I started the OpenVPN server on 1594 UDP port, generate certificate and export configs for openVPN client. But I am not able to connect to OpenVPN client. I am missing major configs, like where do I provide my ISP router's WAN address (or public IP address). Also, once this is done I will need to configure DDNS server too, I am trying with some free options to start like no-IP with Let's encrypt cert, will

Question by Themane on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.