HTC Auto switch WiFi/ISP

[fifty1ford]

OK, I must be missing something but I can’t seem to find what I’m looking for.
I have a HTC 6800.

When I have my WiFi switch turned to ‘on’ I want my phone to use WiFi for email/web.
When it is off, I want to use my ‘ISP’/cell connection.

Seems simple, but how do I make it happen?
I’ve been searching this forum and the web for a couple of hours now…

I don’t want to have to go change settings in ‘Network Management’ every time.
Also, using the ‘ISP’/cell connection automatically if WiFi cannot connect would be great, but I’d settle for it simply recognizing that when WiFi is off it should use the cell connection.

Is there a setting I'm missing or a program for this?

Thanks!