How do I whitelist on the xfi router

I have devices that xfi's advanced security are blocking and I want them unblocked. One example is my smart thermostat. I enabled advanced security and it told me the thermostat was going to a known malicious site. The address listed was internal to my home network btw. I want to whitelist devices but there doesn't seem to be that option. I am incredibly uncomfortable with the idea that Comcast tech support has more control over my network than I do.