This code overrides the "URL redirect after e-mail activation" setting. After activation the user will be redirected to the page used for registration. This may be helpful if the registration page is placed in a popup or sidebar.

um_after_email_confirmation_redirect.php

<?php

// Save the page URL on registration.
add_action( 'um_registration_set_extra_data', 'um_registration_set_extra_data_custom', 10, 3 );
function um_registration_set_extra_data_custom( $user_id, $args, $form_data ) {
  if ( isset( $_SERVER['HTTP_REFERER'] ) ) {
    $url = esc_url( wp_unslash( $_SERVER['HTTP_REFERER'] ) );
    update_user_meta( $user_id, 'um_registration_page_url', $url );
  } elseif ( isset( $_SERVER['REQUEST_URI'] ) ) {
    $url = esc_url( wp_unslash( $_SERVER['REQUEST_URI'] ) );
    update_user_meta( $user_id, 'um_registration_page_url', $url );
  }
}

// Redirect to the saved registration page URL after e-mail activation.
add_filter( 'um_after_email_confirmation_redirect', 'um_after_email_confirmation_redirect_custom', 10, 3 );
function um_after_email_confirmation_redirect_custom( $redirect, $user_id, $login ) {
  $url = get_user_meta( $user_id, 'um_registration_page_url', true );
  if ( ! empty( $url ) ) {
    $redirect = add_query_arg( 'umuid', uniqid(), $url );
  }
  return $redirect;
}

This gist is a part of the article Redirect to the previous page after e-mail activation.

Documentation: https://docs.ultimatemember.com/
Support forum: https://wordpress.org/support/plugin/ultimate-member/