Enabling HTTPs and 3rd party SSL certificate

All of the QNAP NAS across multiple branch offices and my customer locations do not have an on-premises Active Directory connection or AD DS / LDAP server.

When enabling the below feature on each of the QNAP to enforce HTTPS, do I need to buy 3rd party SSL certificate ?

Are you exposing your NAS units to the internet directly? It’s not at all recommended. If you are using a VPN to connect everything, there’s no real need for HTTPS.

Your browser, and sometimes other programs will gives you warning about it if you don’t have a valid (means issued by a trusted certificate provider) certification when using HTTPS connection. But usually the warning can be ignored if you ensure the NAS is in your LAN.