I turned on my TV and clicked the Channels DVR app on my FireTV 4k and I get no OTA. I have checked the signal and it's fine. I can use the HDHR app and watch the OTA channels just fine. I updated to the latest version and rebooted the Synolody DS-718+ that's running the server software. I also rebooted the FireTV 4k. The TVE channels are tuning in just fine.
P.S. I just tested with my Nvidia Shield and it tunes the OTA channels just fine.