Hello,
I am a bit troubled. How does Concrete5 manage to even work on an asp server?
Either PHP is present and C5 works and "asp server" here is not the issue. Or only ASP is present on the server and C5 can't work at all.

Another option is that I'm overlooking something important :)

C5 works ok on a Windows server providing PHP is installed of course, I used to run multiple C5 sites on a Windows server. It's when dealing with URLs etc that things get complicated.

Windows doesnt use htaccess so I had to install IIS URL Rewrite module available on the WPF (Web Platform Installer) to get pretty URLs working and add a web.config file with instructions for IIS to remove the index.php part

As for your issue, I don't know but it might be that it is a URL issue with this mobile add-on.

Sorry yes it is, I just had a look at the email from the hosting company and they said:

"The code cannot work on windows IIS server, since .htacess is apache web server only."

So I can use the mobile add on but need them to do something on the host side?

Depends how much access you have on your server, but you're going to need an alternative that uses IIS url re-write and web.config file to do something similar to .htaccess.
Might be worth asking the add-on's developer to see if they know of a solution.

Do you know if I can put something in the header.php file telling it to send anyone on a mobile device to a particular page and then I can just customise that page with all the info to show looking at a different CSS file?

Possibly, best to Google it and see if you can find some php that will detect the users device and if it's a mobile device then re-direct to your new page.
Or, create a responsive website.