This monitor, and this TV, worked with this server (old Asus motherboard, no on-board video but with a simple VGA card plugged in) in the past. The server was shut down for several years (personal medical issues, I wasn’t up to addressing a server boot issue and wasn’t using the files anyway). (I ran the video to the monitor and to the TV on VGA before, same cable, and it works in the BIOS and early stages of boot.)
Fixed the server boot issue, pretty sure (the drive lights look just like a running server). (The CR2032 battery needed replacing, then I had to make sure secure boot was turned off.) Obviously I’m rather paranoid about any risk to the data zpool (it’s doubly redundant, I see no signs of an issue, but paranoid).
However, it doesn’t come up on the network, and it doesn’t display video past the early stages of boot. (The BIOS displays on the monitor/TV, and some early stages of boot do as well, but before I get to any of the things I need to know, the monitor or TV is displaying its version of “I don’t know this resolution”.)
It responds to CTRL-ALT-DEL from the keyboard to shut down, if I’m interpreting the disk light activity correctly.
The Software version will be a couple of years out of date, at least, but since it worked on this exact hardware before that’s low priority right now.
All the advice on fixing video issues seems to assume network access, and all the advice on fixing network issues seems to assume console access 
. No IPMI on this MB, this is a cheap home setup.
I’m hoping for somebody sufficiently cleverer than me to suggest something simple that at least gets more information.
My one idea is to replace the VGA card with a simple video card with HDMI out. The TV has HDMI in, so that should get me around the VGA resolution restriction I seem to be encountering (which was never there before? But it’s there now.). So maybe what would be useful is – how do I identify a minimal (cheap) graphics card that will work with TrueNAS? Can I plug in any random old graphics card I have?