Monterey - Updates stop downloading at end

Hello,

I have a MacBook Air (13-inch, 2017). I was running Bootcamp on it, but have just removed the partition for it and have erased the disk.

In Recovery Boot, I re-installed the OS that came with the computer, Mojave.

Then I did the security and Safari updates that Software Update found automatically.

Now though, I'm trying to update to Monterey using Software Update, but when the updates have finished uploading the bar gets stuck a bit before the end. When I close Software Update I have to start downloading the updates again by clicking Upgrade Now. This has happened many times.

I've tried installing Catalina from the App Store, but the same thing happens.

I've also tried everything on this page: If an error occurred while updating or installing macOS - Apple Support, but it hasn't helped.

There may be an intermittent fault on the phone line, but the phone company can't find out what the problem is and most of the time the internet works okay. I managed to download Mojave and its updates okay, so I didn't think this could be causing the problem, but I'm not sure. Sometimes when I download the updates I get the error message "The Network Connection was lost" but not every time.

If anybody can help, I'd be grateful.