OS X Base System only disk available - unable to restore high Sierra

I've got an iMac 2013 and trying to factory reset the HDD to wipe it clean.

I got into disk utility and erased Macintosh HD then I restored/copied OS X base System to Macintosh HD. (I realise this was incorrect)

I've been able to erase the internal "OS X base system" via the erase function by using "option-command-R" but now when I go to "Mac OS utilities - reinstall macOS" ( high sierra) - even with active internet connection - I get the error "recovery server could not be contacted"

Any thoughts on how I can re-install High Sierra?

Thanks!