Since there is a snapshot build available, I doubt that one really needs to build from source for the v5.
git checkout v18.06.0-rc2 looks a bit outdated.
Installation instructions that should work (Source):
Currently the only method to install openwrt for the first time is via
TFTP recovery. After first install you can use regular updates.Flash instructions:
- To flash the recovery image, start a TFTP server with IP address
192.168.0.66 and serve the recovery image named tp_recovery.bin.- Connect your device to the LAN port, then press the WPS and Reset
button and power it up. Keep pressing the WPS/Reset button for
10 seconds or until the lock LED is lighting up.
It will try to download the recovery image and flash it.It can take up to 2-3 minutes to finish. When it reaches 100%, the
router will reboot itself.