Whatsapp media deleted / won't restore

My iPhone 6S 128GB was stolen on April 9.

On April 10 I got a new SIM card with my old phone number and started using my old iPhone 5 32GB with same iCloud login. When I tried to do an iPhone restore from backup, there wasn't enough memory to do a full restore, so I temporarily tried an iCloud backup which barely backed up anything, no photos, no apps, nothing on the iPhone 5.

When I tried to use Whatsapp, it informed me I needed to update the operating system, so I did (I assume to the latest). Then I restored Whatsapp in the normal way. I don't remember at that point how much of the media restored to it, or whether the old media was there or not. The messages were all there though.

Used it for a couple of days, then moved the SIM to an iPhone 6 and restored from a March 4 iTunes backup with same iCloud password. Again, restored the Whatsapp from iCloud backup and the messages were there, but I eventually noticed that the media was not there.

A few days later I switched on the old iPhone 5S to look in Whatsapp and indeed the media was also not there. So it seems that either the media never backed up through iCloud at all, although I have daily chat backup in Whatsapp and backup in iCloud. Or that something happened when I tried restore on the iPhone 5, which deleted the media in iCloud. But according to Whatsapp I have sent about 1GB and received about 3GB (this has never been reset), so with a total of 4GB it shouldn't be a storage issue even in a 32GB iPhone 5S, especially with no Camera Roll photos restored to it.

At that point I unhooked my Apple ID from the 5 and erased it in case it was an issue of being in two devices at the same time. And then I restored Whatsapp in the 6 again.

The weirdest thing is that I DO have most all media except one or two videos, from April 6 to today April 19th, which can only have come from iCloud backup, as I hadn't done an iTunes backup since March 4, and my phone was stolen in the middle of those dates. So some of the iCloud backup did work.

The only thing I can think of is that when I started using Whatsapp on the old iPhone 5, it backed up whatever media it could until April 6, then stopped (because it was full?). And then I did a backup of the Whatsapp chat inside Whatsapp itself which was already missing many files, and it wrote over all the media that hadn't been restored earlier than April 6 with blanks, affecting all future backups of Whatsapp in iCloud no matter what device or what version or what backup.

Because even when I restored from March 4 iTunes backup, which theoretically shouldn't be affected by what I did later, the media was all gone.

Any advice? Is there anything I can do or try? Or should I just give it up. I've read every thread, and I think the last resort might just be one of those recovery software programs, but have doubt even that would work.