Missing photos from before December on iPhone

If you deleted the pictures, then deleted pictures remain in the "Recently Deleted" view for 30 days, and then they are removed permanently. This safety feature can be defeated by choosing to prematurely delete pictures from the "Recently Deleted" view. If you use iCloud Photos, then that "synchronizes" its Photos Library with the phone, so what you see on the phone is what is at iCloud. iCloud Photos doesn't keep pictures in "Recently Deleted" any longer than the phone does.

Some pictures transferred from a computer can be viewed in Photos but they can't be modified by Photos, so there may be some operations that produce a warning about those being at risk.

You can see this:

         If you're missing photos or videos in the Photos app - Apple Support

Basically the above says to look in Recently Deleted and in Hidden. Be sure to check your Library thoroughly-- the pictures may not be where you expect, as dates may have changed. You can use the search field with the date or location. And remember, you can sort the Library by date captured or by date added. Also, your Library view now has filters, and you may not have your filter set to "All Items:"

If you don't use iCloud Photos, but you have Settings>Apple ID>iCloud Backup turned on, then you may be able to recover data from the last 30 days or so, but you would need to look at the date of the backup to be sure it's from before you lost the images. You should look here:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support 

This is what you do when you buy a new phone, so this safety, if you have it set, requires you to erase everything on your phone and replace it with the backup.

If you don't keep your own backups to an external drive, then there really isn't any place else to look.