Well, your receiver clearly supports 4k HDR @60Hz so you should not have any issues whatsoever unless you somehow have a faulty input or substandard or defective HDMI cable. Your cable must be able to handle at least 18GB of signal pass through.
http://us.marantz.com/us/products/pages/ProductDetails.aspx?CatId=avreceivers&Pr oductId=SR6010
Prepared for the future, the SR6010 features an advanced video section that's fully compatible with the latest HDMI 2.0a and HDCP 2.2 specifications on all 8 HDMI inputs. With 4K Ultra HD 60Hz video, 4:4:4 Pure Color sub-sampling, High Dynamic Range (HDR) and 21:9 video, 3D, and BT.2020 pass-through support on every input, the SR6010 is ready for the next generation of Blu-ray disc players, set-top boxes and other 4K Ultra HD sources. Certified by the Imaging Science Foundation, the SR6010 features a full suite of video calibration controls (for use by an ISF technician), along with ISF Day and ISF Night video modes.
I would suggest if possible that you plug the ATV directly into an HDMI input on the side of your OLED. If you get a clean picture on a known HDR 4K title with which you’re familiar, then you know you have an issue with your receiver- perhaps the input. Or it could be the cable. Which cable are you using? And I’m not sure what you mean when you say “And the Apple TV will not work with the Ultra HD turned on.” one either selects HDR or SDR so sorry, but I’m confused. Are you referring to receiver setting? If so can’t help you there. :-).
Bottom line, if you get a clean picture streaming SmartTv HDR 4k then it is likely an issue with your cable or HDMI input on your Marantz. Again, if the issue resolves after plugging directly into TV with the SAME cable then you have a bad input or possibly a defective ATV you should exchange.