Currently, softether VPN is not directly supported. However, there are some packages available from opkg that can potentially work, but I am not sure. Check for the packages from CLI/SSH:
opkg update
opkg --size list | grep softether
The package will only be available from the CLI and there is no WebUI implementation. I recommend you read about the packages here.
Not sure if this will fit to your device as RUT241 is quite limited on RAM and Flash. If you have a RUTX series device, I suggest you try this on that device.