Will downloading this plugin help to fix this problem or do we need to take further action?
You need to talk further action.
The plugin helps to prevent some type of attacks, by providing more visibility to the user about suspicious activity generated by non-regular web traffic. If you are already infected, what you need is a malware clean up service to flush the infection from the code/database. Then, you will want to install a web application firewall in front of your website to prevent re-infections.
We would be very grateful for any advice on how to fix this problem
If I understand correctly, the redirection only happens when you click a link from Google, but the website itself works okay if you type the domain in the address bar, right?
In that case, the infection may be inside the access control file, commonly known as “htaccess”. There is a type of malware that checks some information in the HTTP request, generally checking if the user-agent is one of the popular web crawlers like Google, Bing, Yandex, etc. Then, it redirects the user to another website.
Locate and inspect that file, you may find the infection there.
If that doesn’t solves the problem, there are a million of other things you can do, one more complicated than the other. It would be better to hire a security export and let them deal with this problem. There are many companies offering these services, Sucuri is one of them [1] if you are interested.
[1] https://sucuri.net/website-security/malware-removal/
