Previously synced photos not deleting
I synced photos on my iPhone with photos from my computer on an old Photos library (now only on an external hard drive) and now that I've had a new library it won't let me delete these photos in iTunes even if I click and unclick sync photos and click remove photos as in the instructions below:
To delete synced photos and videos from your device
In iTunes, click the Device button in the upper right corner. (If viewing the iTunes Store, click the Library button in the upper-right corner first.)
Click the Photos tab in the resulting window.
Choose "Selected albums" and deselect the albums or collections you want to delete.
Note: To delete all synced photos, deselect "Sync Photos from" and, when asked, click "Remove photos."
Click Apply.
I've also tried going into image capture and deleting the photos manually there as well but they do not show up here.
Is there anything else I should try?
iPhone 6, iOS 11.2.1