The Recovery Server Could Not Be Contacted

Before I start - I am THE least technical person to ever exist in this world. Please forgive me.

I have given my iMac (2010) to somebody and they’ve given it back to me to sort as they weren’t able to connect to anything (something to do with my details somewhere?)… I have deleted a disk I think it is (I saw it on a thread on here) and am now trying to reinstall macOS High Sierra but it’s constantly coming up with ‘The recovery server could not be contacted’. So far I have;

Checked it’s connected to WiFi

Checked the Apple server

Have re-deleted the disk (selecting APFS encrypted instead of just APFS)

Checked time and date

Everything I’ve checked is as it should be but I am still getting the same message about server not being contacted. I’ve restarted a couple of times, too.

Please, please could somebody tell me what I can try as I really am at the point I’m going to throw it in the bin. I’m not made for this 😂

Thank you.