Not able to contact server (web/mail) when connected to local network

Hi,

My internet provider changed their services, so now I am not able to connect to mail and my hosted webpages when connected to the same local area network as my server. As soon as I move outside of my LAN, I have no problems.

My provider suggested activating UPnP, but that does not help at all. Is there anything I can do to solve this problem?

I have DMZ host to my local server ip address.