Reeder on the iPhone syncs with Fever, but neither the Mac nor iPad versions of Reeder do.
Sunstroke (https://goneeast.com/sunstroke/) is a better Fever-specific client for the iPhone, I think... but it doesn't have either an iPad or a Mac version. On the Mac you can use Fever's web interface but on the iPad you're pretty much out of luck, as there are no native clients and trying to use the web interface there is crazy-making. (Actually, the web interface is a little crazy-making in any web browser. It's just a lot crazy-making on the iPad.)
Sunstroke (https://goneeast.com/sunstroke/) is a better Fever-specific client for the iPhone, I think... but it doesn't have either an iPad or a Mac version. On the Mac you can use Fever's web interface but on the iPad you're pretty much out of luck, as there are no native clients and trying to use the web interface there is crazy-making. (Actually, the web interface is a little crazy-making in any web browser. It's just a lot crazy-making on the iPad.)