how to PXE boot on Minnow Board

Question

I try to boot PXE on Minnow Board, which is board for embedded.

Server is Ubuntu.

1. Install tftp server and confiure
2. Install dhcp server and configure
   1. transfer bootx64.efi to client as NBP file.
3. copy bootx64.efi, grub.cfg, initrd and vmlinuz to tftp root directory

Client(Minnow board)

1. Configure boot order to run PXE boot firstly
2. Push Reset switch
3. DHCP works correctly and success to download bootx64.efi file.
4. But GNU GRUB is displayed, I could only type GRUB command and not to boot.

Question

1. Do I have any mistakes about this steps?
2. I think, if grub.cfg is read successfully, boot menu is displayed. Could you tell me how to run boot sequence correctly?