You need to flash the V3 image in Luci if you have used V4. Uboot doesn’t appear to work when downgrading from the V4 snapshot.
Try resetting the router by pressing the little reset button, then connect to ethernet cable and manually assign the ip “192.168.8.100” (i believe default IP range starts there, might be 150), to your PC, and set default gateway to “192.168.8.1”.
Then try to access it from the web. If able, flash the latest stable firmware through LuCi in system - backup/flash firmware.
Do not keep settings, enable the “force flash” option.