Access to 3g usb modem admin web page for VPN client

Hi all. I have a simple question:
How to organize access to admin web page of USB 3G modem for clients connected via VPN?
In firewall settings for the VPN zone, forwarding to WAN zone is allowed.
The modem assign to WAN firewall zone.
The client can ping IP( modem successfully, but if try open web page in browser - there is error:
"Access Error: Site or page not found" (