i run oscam on pi but one problem..

Or if you have at least one computer or router in the Internet (public) network, you can run OpenVPN-server on it. Then you connect with set-top boxes also with OpenVPN-client. These devices can then communicate over the virtual network so created.

Sorry, but I still don't know what your ISP has to do with this.

1.) Port forwarding must be used on each connection node, that is, also on your WIFI router ... it serves to route network packets to the destination, through individual networking nodes

2.) The ideal way to connect multiple LANs over the Internet is to add them to OpenVPN but in this case there must be at least one OpenVPN server on the internet (public IP address) to which each device can then connect without problems ... these devices then communicate via VPN (Virtual-Private-Network over the internet -/or/- virtual LAN over the internet) - but at least basic knowledge of computer networking is required

3.) If your WiFi-router is connected directly to the internet and owns a public internet IP address, then just use the DynDNS, DDNS, DynamicDNS, or similar service called ... which is a service that is unfortunately usually charged - but not always, for example, ASUS company is adding to their own routers the possibility of a free "DDNS account". This way you always connect only to the hostname (to the DDNS name in the internet) and this DDNS service, since your internet IP address assigned by the provider for your router can change at any time, so this DDNS redirects the requests always to your router's IP address, after changing it. This is provided by the DDNS service on the router, which constantly communicates and always re-registers your router's new IP address on the internet. You always use the DDNS hostname in the internet, instead of the dynamic IP address of the router (which always directs the communication to your router).