Pretty sure Signal is funded by CIA and intentionally makes what you’re asking difficult, maintaining their belief that only an Android device can have security - and a Librem 5 has not security, and mustn’t be supported - leaving you to fend for yourself.
As was linked above, there are a couple of options. You can move your Signal account to signal-cli, a fork that iirc changes the “libsignal” part to remove the Google spyware (sending metadata for each message to Google), then they package the modified “libsignal” as a command line app.
Of course, according to a Signal rep a few years ago, it is against Signal terms of service to use a fork of their “open source” application, but my text below is written assume you don’t care.
Once your account is connected to signal-cli instead of Android, then you can link your “desktop” Librem 5 against the signal cli. But normally linking desktop requires scanning QR code so you obviously have to overcome that. I haven’t used Signal-cli but the Molly app for Android has a backup option to enter text instead of QR code. But Molly is not available in most places because Signal used cries of “it violates our ToS to fork our code” to try to destroy availability to it (for being a fork). So whether you use signal-cli or Molly you can imagine it might connect you up. But don’t make a mistake when transferring your account over - for example, don’t use a Molly or signal-cli version that is more than a few weeks old - or else Signal servers will ban you for 7 days from using Signal at all.
So then when you have transferred your account to a super secret not-allowed fork, you can use Flare or Signal-Desktop or something similar to be your Librem 5 app for doing Signal. Of course, Flare is subject to the same risks that Signal might ban you for using it if you don’t use the latest version, so don’t try to apt install flare or whatever. Instead, using some flatpak or something that does frequent rolling updates (and you have to get your flatpak from some third party source, not a trusted source like our PureOS flatpak hub, since Signal team makes sure PureOS and others will not work, by doing their frequent updates.
So if you choose Flare then the problem is calls or group messaging probably won’t work or something, because some honorable volunteer bothered to remake it all from scratch. But if you use Signal Desktop, obviously for a year or two the main way to obtain that was from an unofficial arm64 build noted on Purism forums. And that unofficial build’s maintainer left a footnote: “I no longer use Signal.”
So it’s kind of a head-scratcher why the one guy who knew what’s going on didn’t want to do it anymore.
But then, I mostly stopped using Signal, so my info is probably out of date.