To remotely access your Raspberry Pi using SSH, we only need to add a rule to your router so that it can route the incoming ssh traffic to your Raspberry. In particular, it will route incoming traffic at port 22 (default SSH port) to the SSH port of your Raspberry Pi.
Go to your router portal (e.g. 192.168.1.1) and enter your router admin credentials (they might be in the back of your router; otherwise reach out to your manufacturer / ISP). Got to the NAT section and add the rule.