Have you followed the commands suggested in @ChriChri’s helpful troubleshooting guide? In particular, the lsusb output might shed some light.
I think at this stage few if any are supported by Purism, if that’s what you mean. There’s a couple of devices that Purism may have mentioned enough times to be taken as some kind of unofficial endorsement.
I guess at some point of time in the future Purism may formally do integration testing with some devices and declare that they are compatible with the Librem 5 i.e. officially give endorsement.
Or, who knows, Purism might sell something in the future (whether they develop it or just brand it).
There are a lot of “moving parts” though and it isn’t really clear whether you are asking about a dock, or cables, or a monitor - or the whole combination.
Even “HDMI” is complicated because there are multiple versions of HDMI and because some converters or docks may have limits relating to total bandwidth (i.e. resolution x framerate). (For example, my USB-C to HDMI converter says “4K/60Hz” but not all converters will support that and in fact I haven’t tested it to that limit.)