To downplay the answers given, a VPN on your router is better because:
A) It doesn’t rely on QNAP servers being operational and secure
B) It runs on an edge device in your LAN (i.e. router). not a device already inside with questionable security history (NAS).
C) It provides (secure) access to ALL resources within your home network such as all file servers, printers, scanners, switches, cameras, etc, not just the QNAP device
(Raspberry Pi would be my second choice)
As I said originally, there are a million options well covered by a google search as this is well discussed over many years and is a general, not NAS-specific question.
Also, not knowing your NAS model, firmware version, router or ANYTHING at all about your ISP or LAN setup, knowledge/skill level, or what you are willing to spend in time learning or money makes it impossible to give a proper answer.