If I understand correctly, you've restored back to Windows 8.1 and now want to install Windows 10 via the Insiders program. If so, then yes you can install via ISO, or you can go to
https://insider.windows.com/, login, and kick off the Windows 10 installation from there. If you do the latter, you'll get whatever is being served up initially (10162, I think, since that's going to the Slow ring now).
If you mean you've tried running the Windows Insider install from the site linked above and it's not installing, then I'm not sure what's going on. I know that I've had to try a couple of times myself in the past before it finally "took."