I demand passive cooling for a box of that size, talking about the Librem Mini (obviously not for a fully-loaded rackmount server).
Purism has not traditionally configured the Librem Mini as fanless i.e. the v1 and v2 both have a fan. However I have no information regarding the specs of the v3.
You would need to decide how important fanless is.
If all you are going to do with it is route, say, 1 Gbit/sec between LAN and internet, I wouldn’t expect it to run hot. You might get away with keeping the fan.
If you are adventurous, you could take it apart and make it fanless: Librem Mini Teardown - #9 by morgan
The current Librem Mini versions don’t but I have no information about the v3. You can of course use a USB 3.0 to gigabit ethernet dongle to add a second ethernet port - which is probably adequate for an internet gateway router.
If you had hard requirements of two GbE (or better) ports built-in and fanless there are probably better choices available now but you would be giving up some openness.