Not sure what a "refresh" is exactly, unless you're talking about a "Reset" and choosing to keep you programs, files / data.
And I don't understand how performing a reset would have impacted Windows 10 Activation, assuming that it was activated before.
Perhaps there is an issue with the account you are using or your connection to the Internet?
I have an ISO of both 14951 and 14955 which I would be happy to put up on my google drive if you think either or both might help you repair your computer, but I don't have an earlier build ISO.
I've performed multiple clean installs of Insider Preview builds, including many DiskPart wipe (clean) of the entire contents of the drive before beginning the install and have never had a single issue with activation except when I tried to recover a VM of Windows 10 Insider Preview that had been running on an AMD machine onto an Intel based machine. That was a no go as I recall and I had to start over from scratch as I didn't feel like jumping through any Microsoft hoops to get them to activate it again.