Cannot delete photos from iphone

I agree.. Use any borrowed computer with iTunes or Finder to sync Photos with no selection.

The only other option is to backup, wipe the phone, and restore the backup. Sync’d photos are not backed up.

As far as I know they can still be removed by syncing again without including those photos in the selection of photos to be synced but for some reason Apple seems to have removed all references to that in their support articles about syncing such as Sync photos in iTunes on PC with devices - Apple Support -- at least I can no longer find any mention of it.

You mention not having access to the old laptop, but I don't believe that to be necessary; a sync from any computer, Windows or Mac, will, I believe, remove the previously synced photos but there won't be any way to just delete some of the photos. They will all be deleted (if it still works as before).

Also, the Photo Sync app used to (a very long time ago and if my memory is correct) state that it could be used to delete synced photos but I can no longer find any mention of that either. https://www.photosync-app.com/support/basics/answers/how-to-delete-photos-or-videos

The other option is that if iCloud Photos is turned on in the phone it will give a warning message that the synced photos will be deleted -- but I wouldn't recommend that unless iCloud Photos was the ultimate goal.

Another forum member here -- elcpu -- also mentioned the following:

The only thing I can add to that is to first delete the "iPod Photo Cache" folder if one exists on the new syncing laptop.

I think your diagnosis is correct; photos that have been synced from a computer must be unsynced from it; they are not stored in the same place as photos either taken on the phone or synced from iCloud. I’m not sure how to do that, but I’ll ask around.