You are right because running a Bitcoin core without port 8333 is useless for the network. 8333 is the default port of Bitcoin Core. All nodes in the Bitcoin core are used to communicate and collect data from the network. If any of the nodes here have a problem, it stops the connection of all the nodes. Tick ​​the problem with your ISP. What you want cannot be done for free. Try changing the router settings and forwarding the port to 8333. Although port forwarding can put your computer at risk of hacking, do it cautiously and share it with an expert.