Can I do a 30km wireless link between two LibreRouters?

You can, but you will need to change the antennas.

The LibreRouter is designed with customizability in mind, and the antennas is one of the elements that is interchangable.

The ones that it comes with are sector antennas that distribute the power in a 100° beam.
Narrower antennas will focus the energy more and allow you to get further away.

As an example, we tested two LibreRouters with a 34db MIMO 2x2 dish antenna on each side and we accomplished a 40km link.

(image pending)

Though we achieved a link, we could transfer 4Mbps through this link.
The reason why this happened was because the spectrum was crowded of other WiFi devices. As the LibreRouter currently follows the 802.11 protocol that does not allow a device to transmit if there is another device already transmitting then the LibreRouter couldn’t use the full airtime as most of the time that it wanted to transmitt it could not because another device was transmitting.
The LibreRouter doesn’t implement any TDMA protocol that for long range links is a workarround arround this high airtime usage by other devices. When a free software implementation of TDMA is available we will provide it with a software update.