Hello Pfann,
So if I understand correctly you have an OpenVPN Access Server running somewhere on the Internet with a public IPv4 address. You want to make it so that when you access that public IP address from anywhere on the Internet, that that request ends up at your PC that has an LTE internet connection.
To do so, go to the admin web interface of the Access Server and go to User Permissions. Look up the user account that your PC with LTE is using to establish a VPN connection to the Access Server. In there enable the DMZ functionality. Type in the IP address followed by a port. For example if your OpenVPN Access Server's public IP is 123.45.67.89 and you want to open port TCP 80, type: 123.45.67.89:tcp/80
Save settings and update running servers. Now when port TCP 80 is addresses on 123.45.67.89, it will be redirected to your PC with LTE connection.
Some caveats; the IP you specify here must be an IP on the Access Server itself. If this is on Amazon AWS for example you have to use the internal IP of the VPC because that's the IP that the Access Server really has - the public IP is just NATted to that. Also whatever service you're running on your PC with LTE must be configured to listen on the VPN interface. And firewalls must be adjusted to let this traffic through.
See also this guide;
https://openvpn.net/vpn-server-resources/how-to-setup-dmz-in-openvpn-access-server/
Good luck,
Johan
