HDMI extender works for 20 seconds, then goes blank

Given that the issue occurs after exactly 20 seconds and the display returns with a resolution change, it's possible that the problem is related to the wireless HDMI extender's ability to maintain a stable connection at the resolution set on your MacBook Pro.

Here are a few steps you can try:

• Although you've tried resetting the resolution, it's worth experimenting with different resolution settings beyond just the default one. Try lowering the resolution to something lower (like 1080p or 720p) and see if the extender works consistently. Sometimes, high resolutions (4K, for example) may exceed the wireless HDMI extender's capabilities, leading it to disconnect after a brief period.
• Check if there are any firmware updates available for your HDMI extender.
• Often overlooked, but wireless extenders can be sensitive to interference from other wireless devices. Try moving the extender or your MacBook Pro to a different location, away from routers, Bluetooth devices, or microwaves. You could also try changing the wireless channel or frequency on the extender (if that option is available).
• Although you've tested different displays, it might be worth testing the extender with a different laptop or desktop to see if the issue persists. If it works with another computer, the issue might be related to specific settings on your MacBook Pro, such as graphics drivers or display preferences.

Finally, if all else fails, it might be worth reaching out to the manufacturer for further troubleshooting or to inquire about possible hardware defects.