Hello,
Thank you for contacting .
On the RUT240 latest firmware version 01.13.1, you need to enable IPv6 support in the menu System > Administration as you already did.
Then you can adjust settings on the Web UI menu Network > WAN > Mobile (Edit).
By default DHCPv6 server is enabled. You can also make changes on the Network > LAN > General Setup page.
I have also attached some screen shots for your assistance.
I hope it helps .
Have a good day.
Regards,
Ahmed