Set account to pending after activating account via email link

If you set the Registration options in wp-admin>Ultimate Member> Settings> Email> Account Activation Email to send activation email links to users, they will be automatically approved.

The sample email activation.

So this means you need a custom code to make the user's account status to 'Awaiting Admin Review' after clicking the activation email link. You can try the following code below:

When the user tries to log in while the admin hasn't approved the account, they will see the message like in the image below.