Hi Andy,
Thanks for the clarification.
Please can you confirm the controller platform and software version you are using.
The data sheet for the R510 indicates that the platform is capable of using the higher frequency channels:
However it could be a limiation on the version of your controller platform and/or version. I've attached a screenshot of SmartZone 5.2.0 on a UK-configured Zone, with 5.8GHz channels enabled.
Thanks,
Darrel.