Both u-boot and the kernel can set up wifi and networking.
That is how it "network boots".
Watch your wireless router for a connection attempt (you might have to set ip= in the kernel command line).
Since the only way to communicate with u-boot is via its serial port - -
You are sort-of SOL without the serial cable.
Ref:
https://wiki.mobileread.com/wiki/Hard...ex#Serial_Port
Read the first caption line to that section please.