Thank you for your assistance.

I am using an AXE5400 at home and would like to access my NAS on my home LAN via IPv6 OpenVPN from an external client PC.
Can I allow external communication by adding the following information in the ACX5400 management screen under Advanced Settings >> IPv6 >> Add Firewall Rule?

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Service Type: Custom
Service Name: Any string
Internal IP: Specify an internal IPv6 address
Port: 1194
Protocol: UDP
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Thank you for your confirmation.