Extender network mode should work, with the following caveats:
You should use a Static IP address for the NAS, or it may be necessary to unplug and replug the LAN cable for the Beryl to pick up a DHCP IP address from the main router.
The Beryl LAN IP address must be in a different subnet than the main router and NAS. The Beryl LAN IP is only used to access its Admin Panel.
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