1. In some distributions support for NFS root is not in the default kernel. Which distribution are you using? Can you check the kernel configuration (which version?) whether it’s enabled?
2. And the kernel must be configured for automatic configuration (most likely DHCP) or get the address already assigned on the boot prompt. It depends on the boot loader how to do it – which one are you using?
NB: I did this 10+ years ago by getting the kernel from a floppy and then use an NFS root which is a similar setup to yours.
[Update] And the driver for the network interface must be available already when the kernel comes up. Hence it must be included in the kernel or the initrd.