I have not tried File Sharing specifically but for general remote access to my router and VNC into my PC from outside home LAN network, I am using ZeroTier over my OPenWRT device (not a GL.iNet device). Instructions at Home · mwarning/zerotier-openwrt Wiki · GitHub .

ZeroTier create a separate private LAN network that tunnels (like a VPN) over the WAN connection of a device connected to a ZeroTier network. It works even over NAT, atleast in my case.