Then set up port forwarding on your router. To do this, go to your router settings and:
1. Set up a DHCP reservation for the server. This should be easy enough, but depending on your router you might have to look up the server's MAC address and set it manually.
2. Set up a port forwarding connection from the router settings. The internal IP should be the one you just reserved, port 22, and the external port should be pretty much anything but 22; I recommend choosing a random number from 10000 to 65535.
3. Now you can use it anywhere! Look up your public IP (https://duckduckgo.com/?q=what+is+my+ip&ia=answer) from your home network, and SSH into it like this:
ssh <username>@<public ip> -p <external port>
You can then access port 80 of the server securely with
ssh <username>@<public ip> -p <external port> -L <some port>:localhost:80
I've glossed over the details quite a bit; the dynamic DNS service will probably have more detailed documentation. But basically what you're setting up is:
1. Your router will determine its public IP regularly, and tell the dynamic DNS service what it is.
2. The dynamic DNS service will update the mapping from your domain name to the correct IP address.
3. You'll be able to use your domain name instead of the IP address in the ssh command. Very handy if your IP changes when you're not at home, and can't look up your public IP.