It may be that the problem came when you exported the pictures, and the original image creation dates were lost in the transition. The image creation date is stored in the Exif metadata, and if the date taken is still in the metadata, then Photos will recognize it. If those creation dates are lost, then there's no hope, unless you have copies from before that transfer. You can also check your pictures from the thumb drive, outside of Photos, with the free app "ExifTool Reader," to see what's actually there in the metadata.
But make sure-- check the Info Window, ⌘-i, to see if the date is the date taken rather than the transfer date:
This picture is from 1985, before they invented metadata or even digital cameras (though after they'd invented photography, and well after they had invented dates,) so I had to add the date using the Photos Menu "Image>Adjust Date and Time…" Honestly, while the Adjust function is great for doing one date on one picture, it can be very awkward to do a bunch with different dates. I also included the date in the Title of the picture, and pictures can be sorted by title in Photos.
There are scripts available for altering dates inside Photos here:
Thematic Index to Photos for Mac related … - Apple Community
The one I use with old pictures takes the date of the first one and applies that to the others with incremented time intervals. I apply this after I custom arrange a bunch of pictures that were taken together.
I and others use the 3rd Party app GraphicConverter ($40) to edit metadata in batches before importing to Photos. There are apps in the App Store that allow editing metadata, but they all seem to have the dreaded "in app purchases."
The very best deal would be if you can find the actual Photos Library that those pictures were in originally.
