I have a 600/600 fiber connection and i have no issues uploading with around 600mbit but it seems as download is capped at around 230mbit. Hw accell is enabled.
Make sure that under LAN, Switch Control NAT Acceleration is enabled.
Then verify that Tools, Sysinfo, HW Acceleration is enabled.
You may also need to turn off your modem/ont and/or your router for ~10 minutes or more to reset the ISP connection properly.
Are you running any special services on the router? Is an USB HDD attached?
I would also upgrade to 374.42 and if the same issue occurs, do a complete reset to defaults, manually setup the router (including new ssid's and don't use a restore backup file) and possibly test for the best channel on each band.
You don't say if you're testing with a LAN (wired) connection or with the WiFi - but I'm guessing LAN if the upload is 600Mbps.