i have a vigor 2860n router, and huawei E3531 usb dongle installed with sim card. i have enabled WAN3 in DHCP mode, but only a local IP address is visible for WAN3 in the dashboard page. The address is 192.168.8.100. as it happens the setup page for the dongle is 192.168.8.1, so must be usingt hat network address. The led status indicator for the dongle does go solid blue to indicate it is making a link to the public network.

before installing it in the router i set it up on my laptop, and it picked up a public IP address Ok & i could access the internet.

any ideas what could be wrong.

Some dongle I think you cn set in bridge mode, failing that, for inbound connections, use DMZ.

Who is the provider? Most of the mobile ISP's present a CGNAT connection regardless. If you use three, you can get a public IP by using "3internet" as the APN.