Based on the screenshot, you are in the correct configuration page.
Could you uncheck the Auto APN option, enter custom APN, according to your carrier and see, if you are able to receive IPv6 address?
Otherwise, I would like you to attach a troubleshoot file to your question. Please, replicate the issue, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. The logs in the file might provide more insight into the issue.