As you know, client download wdsnbp.com to download pxeboot.com and then download other files.
You can view blow article to know usual boot process:
troubleshooting-sccm-part-vii-osd-part-i
The wdsnbp.com file is the file on the DP after we enable PXE on DP. The file path is: C:\RemoteInstall\SMSBoot\x64 or C:\RemoteInstall\SMSBoot\x86. Although we don't configure wdsnbp.com in the task sequence, this is what the boot process needs.
On your question, It seems that the wdsnbp.com file could not be found.
You can check the following to try to resolve the issue:
1). The boundary group is configured correctly. Make sure DHCP-scoped boundaries, mp servers, and DP server(DP for PXE is enabled) in a boundary group.
2). WDS is not configured successfully. If WDS is not configured successfully, there will be no wdsnbp.com file.
3). IP Helper was unable to locate the pxe-related file.
If the response is helpful, please click "Accept Answer" and upvote it.