I can see you are unclear on the concept.
When you connect to your ISP, you must follow their rules in order to connect successfully. You can use a static IP if and only if they assign one to you, and they will usually charge you a lot extra to do that. You also don’t need to do it. Static IP’s are for people who are (e.g.) running web servers for the public. For most people, dynamic IP’s are just fine and cause no problems.
IF, however, you have a router, then the router creates a subnet on its LAN side. Over on its WAN side, where it connects to the internet, it too must follow the ISP’s rules or it won’t work.
The router takes the place of your computer, in connecting to your ISP. They say use DHCP, you tell the WAN side of the router to use DHCP.
The LAN side of the router is a different story. You can set this side up any way you want, that works. The way most people do it, and the way it’s set up by default, is to use DHCP to assign subnet IP addresses to anything that connects to it and asks for an IP. These subnet IP’s are valid only inside the LAN. People outside the LAN can’t see the subnet or anything connected to it, they can only see the router.
Inside the LAN, the router has a firewall. If you want to open a port on the firewall, you can only open it to a specific subnet IP address. This is a security measure. So the machine you want to open the port for, must have a static and unchanging subnet IP address. If it had a dynamic address, and that address changed, then the open port in the firewall wouldn’t be pointing to the right machine anymore.
Static IP’s are an issue only if you are behind a router that you control. If you are not, then you cannot use one and have no use for it, and it will not work if you try.
