iPhone AirPlay: Google Photos videos play sound only

When an iPhone video in Google Photos shows a frozen screen and says "connected to AirPlay" while playing audio only, it is usually caused by an iOS or Google Photos media-rendering conflict with the AirPlay video pipeline, often triggered by a recent app or iOS update changing how secure video streams (DRM or high-efficiency HEVC codecs) are packetized over a mirrored link. 

Try this

• Disable Hardware Acceleration / Change Playback Mode: Play the video locally on your iPhone first, tap the-dot edit or cast button, or try playing it directly inside the native Apple Photos app (if backed up or saved locally) to see if the render engine bypasses the freeze.
• Toggle AirPlay Connection: Disconnect AirPlay completely from the Google Photos app, close the app from the app switcher, restart the video on your phone screen natively, and then re-initiate system-wide Screen Mirroring from the iOS Control Center rather than casting directly from inside the Google Photos media player.
• Power Cycle the TVs and Router: Unplug your Roku TV and Samsung TV from the wall outlet for 60 seconds, and reboot your Wi-Fi router to clear corrupted local AirPlay socket buffers.