Unable to contact recovery server during High Sierra reinstall on iMac 2011

Trying to resurrect an old iMac 2011. High Sierra was the latest OS running. Opted for 'Reinstall MacOS'. High Sierra is offered but when I try to continue I get a message 'The recovery server could not be contacted'. Any advice about how to proceed please.

Thanks

Hughie

[Re-Titled by Moderator]

Original Title: High Sierra