I have device that connected to my PC over wifi/ethernet.
After adb reboot bootloader device moves to the fastboot mode.
How can I connect it back with fastboot/ or how can I flash image on it.
I even can't ping my device.
Fastboot devices Should show you your device connected to your PC when its in fastboot mode. If your device isn't shown up then you might lack the adb driver for your device. fastboot -t should be invalid, cause you can "only" use -w/u/s/l/p/c/i/b/n as a parameter for the fastboot cmd.
But whats even more important: fastboot should only work on USB connected devices, however you managed it to connect it via ethernet/wifi.