Transfer videos from PC to iPhone (regularly)

Hi, I need to transfer videos to my iPhone to use for Instagram stories, and this wasn't a problem earlier. But now I find that I can't for the life of me find a way to get them transferred. For several reasons I would like to avoid transfer using iTunes/cable. However, using Airdrop used to work fine (about a year ago). But today I work from a PC, so that's not possible.

My process has been:

Making the videos in Premiere Pro. Saved them as mp4 with H.264 - Matchsource - Medium Bitrate (tried different bitrates).

I have tried to transfer them to the iPhone via:

iCloud

Documents

Dropbox

OneDrive

Google Drive

Bluestacks (emulating an Android phone - it can post it but the video is all green lines)

(probably more ways that I forgot about now).

Always the problem is when I want to save it to the camera roll, then it gets rejected.

Here's the weird thing: I tried opening my Premiere-exported movie in Windows Photos, then choose to "Make video with text" and then save/export it without making any changes, and that works - I can get it on the phone! But the result is of poor quality, even when I choose the highest file size output.

I need to do this fairly often for my work, so I'm looking for an easy way that works. Like save it to dropbox, then add it to camera roll from the Dropbox app. Which used to work.

Any ideas what might go wrong? Is it the video format settings that is wrong, did Apple change the settings requirements for external videos imported to the camera roll, is there a different service that work?

Thanks for any helpful comments,

Alexander