If you have a dynamic IP, you will need to setup a DDNS to access your NAS from outside your local network.
So to start thing off you will need to open an account with a DDNS service provider. There are plenty out there, some paid, some free. The two below are the ones that I use personally. They are free. If you find others then you should also be able to use those. Some of these providers might already be part of the list included by Synology in which case you can skip the 'Customi