To see if Windows is still bootable try creating & using a rEFInd USB stick. When booting rEFInd, it will automatically locate any bootable volumes...perhaps ones which the Apple firmware or macOS won't recognize (both are quite limited & unable to deal with unusual surprises or unknowns while rFEInd may scan & detect bootable systems differently).
http://www.rodsbooks.com/refind/index.html
There is a link to on the following page to download an image file for use with a USB stick. You will need to use the command line to create the bootable USB stick...instructions to do this are in a ReadMe file inside the downloaded file. Make sure to be careful designating the correct drive ID is for the USB stick you wish to use (check in Disk Utility). Also, make sure to unmount all volumes on the USB stick so that you can use the command line to write the raw .img file onto the USB stick. It would be best to disconnect all other external drives just to be safe and make things a bit easier. If you provide the wrong drive identifier to the command, you will destroy all data on a drive with important data.
http://www.rodsbooks.com/refind/getting.html
Otherwise you would need to usb a Windows installer to repair the boot issues of that external Windows drive, but this may not be straight forward since the Apple hardware may complicate things. When booting a Windows installer, there is an option to boot into a set of repair tools...one of which has an option to try to automatically fix common & simple boot issues. If that does not work, then you would need to use the Windows command line to manually figure things out with the Windows installation....that is not easy to do even on a standard Windows PC, but is likely a bit more difficult on a Mac. The last few times I tried to fix Windows boot issues on a standard PC, I was unsuccessful.
Keep in mind maybe the external drive is failing since there are other partitions on a Windows boot drive which macOS may not mount.
