If you have multiple Windows Deployment Servers provisioned in the environment, you will need to enable the WDS server selection feature as below;
Simply go to (this needs to be done on the WDS role running instance):
HKLM\SYSTEM\CurrentControlSet\services\WDSServer\Providers\WDSPXE\Providers\BINLSVC and set AllowServerSelection to 1.
Then restart the Windows Deployment Service. After doing this you get the option to press F11 and the PXE client will now discover local PXE Servers and allow you to select which one you want to use.
Then you will get the opportunity to select the desired WDS instance.