How do I use Signature Version 4 in ordinary AWS CLI calls?

Question

Simple AWS CLI commands like `aws ec2 describe-images --region $REGION` produce the error
`AWS was not able to validate the provided access credentials` when `REGION` is one of af-south-1, ap-east-1, eu-south-1, or me-south-1.

[This StackOverflow  question][1] says that newer regions only support Signature Version 4. How do I choose this in my AWS CLI (v.  2.4.15)? [This says][2] that that happens automatically, but clearly it does not.

Notes:
* This occurs whether I have this  `~/.aws/config file`  or it is missing:

```[default]
region = us-east-1
output = json
cli_pager=
```

* I have no `.boto`. 
* My clock and timezone seem correct.

[1]: https://stackoverflow.com/questions/37873157/
  [2]: https://docs.aws.amazon.com/general/latest/gr/signature-version-4.html

Accepted Answer

Have you [activated those regions](https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/manage-account-payment.html#manage-account-payment-enable-disable-regions)? Those regions are not enabled by default.

How do I use Signature Version 4 in ordinary AWS CLI calls?

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