I tried to simply copy over the pictures, but the gallery application won't recognize them and I did not find a way to rescan the DCIM folder.
Check the question on “Adding a photo directly to iPhone disk”, and its answer which suggests that Photos can be forced to rebuild its library by deleting the following files from
PhotosAux.sqlite. The files that you added also need to be named according to the “IMG_1234.jpg” pattern. I haven’t tested this myself, and the question is a few years old, so this might no longer work on recent iOS versions though.
What other options are available to import photos from PC to iPhone without iTunes, iDrive, iCloud? […] Unfortunately installing apps is impossible due to the restrictions imposed by my employer.
If it's just a few pictures, one option could be to email them to yourself, the Mail app allows saving all pictures in an e-mail (tap and hold one of them, then choose “Save [number] Images”; WikiHow offers a more detailed explanation). Another option could be to use an SD card or camera as an intermediary, though you'd have to get the “Lightning to SD Card Camera Reader” or one of the “Lightning to USB Camera Adapters”.