Emperor-One
commented
1 month ago High Sierra is still available on Apple's install servers. However, as I understand it, the certificate for the download website has expired.
Run this command in the terminal available in the installer:
nvram IASUCatalogURL="http://swscan.apple.com/content/catalogs/others/index-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog"
What this does is it tells the installer to look for the http version of the website instead of https.
Look here for more info: https://mrmacintosh.com/how-to-fix-the-recovery-server-could-not-be-contacted-error-high-sierra-recovery-is-still-online-but-broken/#more-8092
As I understand it, Apple removed High Sierra (and likely anything earlier) from their install servers. So a Nettwork Recovery install of High Sierra will no longer work.
There are various installer images available for downloading, but as yet I have not been able to get ANY of them to work. So I have a nicely repaired 2010 iMac with NO installer to work with it (Catalina Patcher insists I have to do a firmware update under High Sierra, so I can't install it either). I had thought if I could get a MacOS VM working I could at least create an installer USB from there (because it refuses to run under Catalina), but it refuses to work with a version older than your current version.