Today I dug my Gemini, which is updated to Android 8.1, out of storage, charged it up and hooked it up to the living room TV, using the Planet USB-C to HDMI adapter (and a HMDI cable). Technically it did work, but rather than being presented in proper landscape, the image was shrunk down and rotated 90 degrees, like this:
I read
in this thread that the rotation issue can be worked around by disabling the app bar, but if you get no picture at all, that likely won't help you.
If you get no picture, it could be that your particular external screen can't handle the slightly odd 2160 x 1080 output (regular "full-HD" is 1920 x 1080). If it's a PAL TV, it might not like the 60Hz refresh rate (then again, my old TV is PAL, and it accepts a 60 fps signal just fine).
If you get a headphone symbol (I don't), I'm wondering if your adapter or connector might be faulty somehow? I suspect that something as simple as a headset identifies itself by some simple means, such as shorting two particular pins in the connector. Should a pin in your connector (either in the Gemini, adapter, HDMI cable or display device) be bent or otherwise damaged, that could perhaps explain both why you are not getting any image and why you are getting that headphone symbol. That doesn't mean I'm right, it's just a educated guess on my part. There could be any number of other reasons, beyond my comprehension, but you might want to have a look at your connectors with a good magnifier.
