I know this is an old post but I have the same issue with two seperate clients, one has a DrayTek Vigor 2860 and the other a Vigor 2862 and both have single static IP addresses but neither one of them is showing the correct IP on the WAN connection.
The routers are configured with the correct BT assigned usernames and passwords but the public IP does not correspond to the static IP we've been assigned and if the routers are restarted they end up with another random dynamic address which means we then have to reconfigure the DNS at our domain registrar so that our mail server can be contacted.