Navigate to System->Advanced->Networking in webConfigurator.
IPv6 Options
DHCP6 DUID: DUID-EN
DUID-EN
Enterprise Number:
Identifier:
Click Save.
Navigate to Interfaces->WAN in webConfigurator.
General Configuration
IPv6 Configuration Type: DHCP
MAC Address: Same as MAC address of RG
Other options are probably needed, so set those too.
Click Save. This will finally save dhcp6c's DUID file and start the client.
Finished, hopefully.
Good luck!