VPN connection issues with OpenVPN server behind two routers

[Margenius]

Hi people, I bought a GL.iNet GL-A1300 router some months ago to increase the performance of my home LAN. I added this router between provider router (first router, the most external router) and the switch of my LAN. I set up GL-A1300 with a static IP address. I need to connect to my home LAN from outside accross a VPN connection. Even if this router gives me the chance to connect to my LAN home with its VPN, I need to connect to OpenVPN server application that is running in a PC of my LAN.
So, I opened a new port on my first router and configured a DDNS service on it, then I opened the same port on the second router (GL-A1300) and I set up all configuration files on all systems (PC with OpenVPN server and clients like mobiles phones and other PCs). When I try to start VPN connection from outside, OpenVPN client remains in “authentication phase” and after various seconds it appears me “TLS Error: TLS key negotiation failed to occur within 60 seconds (check your network connectivity)” and after it retries to start VPN connection without success!
I think the problem is on my PC, in fact if I try to start VPN connection by my phone from outside and OpenVPN server application is closed, OpenVPN client remains in “Waiting for server reply” phase, but if I open OpenVPN server and after I try to reconnect, application says me usual TLS Error message.
I tried to start VPN connection to OpenVPN server on my PC disabling OpenVPN service of GL-A1300 router, but nothing is changed… Anyone knows how can I solve this problem? Thank you in advance!