First, let me say that this seems to be turning out to be a networking question, and so may get closed or moved. Let me try anyway.
Port forwarding and static IP are two separate concepts.
Port forwarding directs incoming packets with specific destination port numbers to a specific inside address. For example, an incoming packet for port 443 could be directed to a web server. Port forwarding needs to happen at the edge device, often a router or gateway provided by your Internet service provider. In the United States at least, most ISP router/gateways provide for port forwarding. If a packet intended for a particular port can't get through that edge device, it will never reach an internal router that may have port forwarding enabled.
You obviously know about static IP since you're paying for one. For others, it is a registered IP address that does not change, and so can be set up in a DNS server. It is the right tool for the job for public servers. However, for access to a home network, where a small delay is acceptable from time to time, a dynamic DNS service is a possible alternative. A service within your network monitors the IP address supplied by your ISP and, when it changes, notifies a dynamic DNS service provider. When your IP address changes, there could be a small delay in propagation of the changed DNS entry. My home network has an address in the ddns.net domain supplied by No-IP. There are other dynamic DNS providers. Most require a small annual payment.
Once you have access to your home network from outside, it is possible to use a Raspberry Pi as a VPN concentrator. You connect from outside to the VPN concentrator and from that point you appear to be on your home network. A search will find information on this.
Dire warning! Opening your home network to outside access can be very dangerous. Protecting it is beyond the scope of this answer and off-topic for the Raspberry Pi community, but I'd be remiss not to include the warning.
