How to secure a NAS and make it accessible at the same time from any (authorized) Mac connected to the internet?

I have recently purchased a NAS (Seagate Personal Cloud) and have connected it to my home router (Asus AC-68U). I would like to access the NAS from anywhere where I have internet, and at the same time remain secure with regards to unauthorized access and data transfer. So far, I have been using the 'Connect to server...' function on my Mac and the afp:// protocol using my home IP and a port forward on my router. Is this solution secure? Are there any other reliable, secure and relatively fast options, ideally not requiring additional expense, like paying for a VPN?