Ok. You'll need a Sim provider that will provide a 'Public IP' address, not a Private IP address.
You can then use a DDNS service such as DynDns.org and Yes, You'll need to pay a yearly subscription. There are a few free alternatives, that i have not had alot of experience with.
Once DDNS is configured, you can then configure your port forwards from an external port to an internal IP and designated port.
What part of the configuration are you struggling with?
1 - DynDns
2 - Port Forwarding