Something like:
ethtool -s [interface name] autoneg off speed 2500 duplex full
-s is set, interface name you put in yourself as applicable without the square brackets, auto neg off means the links won’t self negotiate speed, speed 2500 is setting 2.5gig, full duplex so we can send/receive at the same time.
Edit: Because I can never complete a single post successfully without an edit - link may need to be bounced afterwards, may not. ip link set [interface name] down to bring down, ip link set [interface name] up to bring it back up.
sudo as necessary.
…Edit 2: you may want to consider either ipmi or physical access to the NAS as if something goes wrong then you won’t have webgui working.
To undo ethtool -s [whatever] autoneg on; once again maybe you’ll need to bounce the link afterwards, maybe not.