It's a TV set, and you drove the HD port, not the computer port.
The TV set user manual is key to a good time, since it has a table of available resolutions, per port.
Regular HD ports expect a "multimedia source", such as a BR set top box. The regular HD expect HD or SD res. Only one port is the "computer" port, and has the rich table of values.
HDMI#1 HDMI#2 HDMI#3 HDMI#4
1920 1920 1920 "computer" or or or 640,800,1024,1280, 1280 1280 1280 1440,1600,1920,"native",...On Windows, you use moninfo.exe from entechtaiwan.com , to read the EDID and display the table of supported resolutions. There is a button for "Real Time" table display, and that allows reading out the table at the touch of a button.
Plug into an HD/SD port, check the EDID, note the truncated table.
Plug into the computer port, check the EDID, note the VESA+ table. The computer port would be multisync, have a scaler and so on, like a computer LCD monitor would have. Some panels support
60 to 72FPS, a 4K panel likely supports refresh at 24 or 29.97FPS or 30FPS, so you can play movies at movie rate.On Linux, "readEDID" and friends, can read the EDID table per port, if you need to query what the hardware support is. If the computer port really wasn't marked, doing a "readEDID" until you found a "rich table of values", would be how you would find the computer port.
For reasons unknown, a decade ago, the "computer" port refused to have a "native value". So if the TV was a better one, the panel on the TV was 1920x1080 say, if you checked the EDID on the computer port, it would go to 1600x1200 perhaps, but it would not have 1920x1080 like the panel had. Since the idiots did not have the "native" resolution entry, this *guaranteed* a fuzzy output. Modern TVs, the scaler goes all the way to "native" or even past that. Some panels could be given a res larger than the set, and scale it to fit the 1920x1080 panel. Modern sets basically do the things you would expect of a computer monitor, on the "computer" port. I would see TVs promising to receive 1920x1080 and scale it down for their cheesy 1366x768 panel (I could never find confirmation this is what they were doing).
But cracking the PDF user manual, is how you learn of the port layout, without getting out of your chair, or needing a flashlight and stuff.
A laptop has modal video, controlled by an Fkey. Repeatedly pressing the Fkey, takes the hardware through the various option. Mirror would be one option. Spanned would be another option. There is likely an "independent" option so the panel runs native and the computer output port on the laptop can be set to 640x480 if you want. YMMV.
PanelOnly, ExternalOnly, Mirror, Spanned, Independent... PanelOnly, ... (repeats)
Some recent laptop CPUs and their video crossbar, have three outputs. They could drive two external monitors plus a TMDS panel. Some desktop designs show four ports on the back, but that does not mean they drive all four ports at once. One of the ports is a cheesy conversion of one of the other ports (HDMI to VGA converter chip perhaps, as the silicon people no longer like to do VGA in their silicon). If you look at a picture of the motherboard, with the heatsinks stripped, you will see a suspicious quad flatpak near the VGA port tower :-) You might plug into the DVID port or the VGA port, but both of those would not operate at the same time. Maybe an Asrock has a design like that. If your desktop has four ports like that, read its manual for usage details and limitations.
Paul