@coondron

You can try this code snippet for verification of the User account ID at login.

Create a new text field in your UM Login form and assign the meta_key um_unique_account_id_login to this field ie your Account ID meta_key from your Registration form with the added _login.

If you have changed the Account ID meta_key in your Registration form’s code snippet update also the code line define( 'USER_ACCOUNT_ID', 'um_unique_account_id' ); in this code snippet.

Install the code snippet into your active theme’s functions.php file
or use the “Code Snippets” plugin.

https://wordpress.org/plugins/code-snippets/

add_filter( 'um_custom_authenticate_error_codes', 'um_account_id_error_code', 10, 1 );

function um_account_id_error_code( $array ) {

    define( 'USER_ACCOUNT_ID', 'um_unique_account_id' );

    $array[] = USER_ACCOUNT_ID . '_login';
    return $array;
}

add_filter( 'wp_authenticate_user', 'authenticate_user_account_id', 999, 1 );

function authenticate_user_account_id( $user ) {

    if ( ! is_wp_error( $user ) && isset( $_POST['form_id'] )) {
        $account_id_login = USER_ACCOUNT_ID . '_login-' . sanitize_text_field( $_POST['form_id'] );
        if ( isset( $_POST[$account_id_login] ) && ! empty( $_POST[$account_id_login] )) {
            um_fetch_user( $user->ID );
            if ( sanitize_text_field( $_POST[$account_id_login] ) != um_user( USER_ACCOUNT_ID )) {
                return new WP_Error( USER_ACCOUNT_ID . '_login', __( 'Account id is incorrect. Please try again.', 'ultimate-member' ) );
            }
        } else {
            return new WP_Error( USER_ACCOUNT_ID . '_login', __( 'Please enter your account id.', 'ultimate-member' ) );
        }
    }
    return $user;
}