The HD621 can do 4k@30 but not 4K HDR, which the OLED TV definitely does. So you need a device to send the full 4K to the TV while sending a reduced resolution to the HD621. An Oppo 203 can do that, but they are ~$700 on eBay: it has an HDMI Input as well as an output. My Roku Ultra is connected to the input of the Oppo. Another option is a device like an HDFury, but I tried this and eventually sent it back. However, to be fair, I was trying it with 4k@60 HDR. It may well have worked with 4K@24 or 4K@30. I might give their 4K Vertex2 a try. I think the fury just got too hot. I'm also seriously considering just getting a 722, but I'm concerned about it working with the playstation. The bits that handle video aren't made by Meridian.

Audio formats are important. Neither the HD621 nor 722 will decode bitstream audio like TrueHD: the source must convert that to PCM. My understanding is that the Apple TV 4k can be told to do that. My Roku Ultra cannot. Would be surprised if your set-top box did. This is where the Oppo 203 comes in: it will decode bitstream signals into PCM. If you use one of the audio extractor ODC references above, you will get, at best, 1.5Mb/s 5.1 DTS, instead of the ~18Mb/s original signal.

Cave: Trinnov Altitude CI-8; Oppo 203 Vanity HD; 6xDSP5k, DSP5kC, D1500; Sony Z9D, PS5 Pro; Apple TV 4K
Living: Wiim Pro; Rega Planar 1+; 2xDSP5k
Master: Wiim Pro; 2xDSP3100
Kitchen: Bluesound Flex stereo pair / Amazon Echo Studio
Deck: Wiim Mini; Random Chinese Class D; Cambridge Soundworks Outdoor 52s; Polk Sub;
Garage: Wiim Pro; 2x5k