Redirect to dynamically generated html in Angular 5

I am building an SPA with Angular 5 and I'm calling an authentication and authorization service. This service checks if I have a session active and if there's no session or if it has expired then returns 401 and an HTML with the company-wide login page. I built a service for making this call, like this:

@Injectable()
export class AuthService {

    private authServicePath = '/<path-to-the-service>';
    private authToken: string;
    private serviceURL: string;
    private requestOptions: any;

    constructor(private http: HttpClient, private cookieService: CookieService) { }

    validateAuth() {
        this.serviceURL = environment.endpoint + this.authServicePath;
        this.authToken = this.cookieService.get('SESSION-TOKEN-COOKIE');
        this.requestOptions = {
            headers: new HttpHeaders(
                {
                    'SESSION-TOKEN-COOKIE': this.authToken
                }
            )
        };

        return this
                .http
                .get(this.serviceURL, this.requestOptions)
                .subscribe(
                    data => {
                        // Do nothing
                    },
                    (err: HttpErrorResponse) => {
                        /*window.document.write = err.error;
                        window.document.close();*/
                    }
                );
    }
}

As you can see, I'm calling the service. If the token is valid then there's nothing to do, but if I don't have any active/valid session, then the service will return the 401 Unauthorized and in its payload (.error field) I will get something like this:

<html>
      <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
      </head>
      <body>
        <form action="..." ...

How do I tell my app to redirect me to this generated page?

As you can see, I tried writing a new HTML with window.document.write but it didn't work. I understand that Angular Router can only redirect to components belonging to the app.

Thanks in advance.

Solution

Ok, so, I found a way to do this. It looks very hackish to me, but it is the first step. If someone has any advice on this, please let me know.

I used Javascript the old way to append the content of the generated HTML to the <body> tag of my app. Since the HTML contains an unnamed form, I've done this:

(err: HttpErrorResponse) => {
                        if (err.status === 401){
                            document.body.innerHTML = err.error;
                            let form = <HTMLFormElement>document.getElementsByTagName('form')[0];
                            form.submit();
                        }
                    }

It looks ugly to me, but it is the best that I can get at this time. If someone has another approach to this, please let me know. I'll mark this question as answered and if there is a better answer I'll be happy to mark it as the right answer.

Thanks