This is caused because the 3dM2 app is using a very outdated TLS 1.0 protocol.
To solve this you need to enable TLS 1.0 while accessing the local 3DM2 web GUI. You should change this back as it presents a large security risk as TLS 1.0 is easily exploited.
To enable TLS 1.0 while using 3DM2 go to "Internet Options" (Internet Properties on newer Windows OS), under advanced check the box for "Use TLS 1.0" then click apply. It's advisable to keep this window open so you remember to disable TLS 1.0 after you are done.
With Use TLS 1.0 checked you should now be able to access the 3DM2 web GUI for your 3ware raid cards using Internet Explorer mode in edge.
Then click more information and go on to the webpage
That's it, you're done.
Don't forget to turn off TLS 1.0 when you're done.
Quick Tip: if you've just installed 3DM2 and don't know your password it's 3ware, be sure to change it.