For the most part, there are two reasons why an internet service provider isn’t broken up for being a monopoly. One reason is that there really is competition, so antitrust laws don’t technically apply. The other reason is that the ISP might be considered a utility or a type of protected monopoly.
There are many things you can do, depending on how the port is being blocked. You can change router or firewall settings to unblock the port. You can contact your ISP to have them unblock it, or you can even mask the port with something like a VPN to trick your ISP into giving you the access you want.
An ISP can only block port forwarding if they control your router, so this is only an issue if they provided the router that you use. When that is the case, you can log into the router and look at the port forwarding settings. If you can’t see those settings, then the safe assumption is that port forwarding is blocked.