OK, it looks like we may have an answer of sorts. Here's a screenie of my Settings/System/About:So, did anybody stuck on Build 10240 figure it out? I'm thinking I might have to use an ISO to upgrade. Settings->System->About shows that my edition is just "Windows 10 Home", nothing about an Insider preview for whatever reason.
As you can see, although it doesn't give the build number, it's listed as Windows 10 Pro Insider Preview.
So, yeah, you'll probably need to install from an ISO.
1. There is no official Microsoft site where you can get an ISO of any of the Builds.
2. There are download sites out there, but even with my knowledge, there's no way I'd dare to take that route. Neither should you.
3. Even if we know of a safe site to get the download from, we cannot post it here because if we do, there will be consequences.
At this point, your only true option is to get the Media Creation Tool from Microsoft. As far as I know, it's only for Build 10240, but a clean install should put you where you need to be to get Insider Builds again.
I've used it to upgrade from Windows 8.1 and it works just fine. Be sure to read everything you can before downloading so you get exactly what you need.
Installing Windows 10 using the media creation tool - Windows Help