I would like to suggest you to get in touch with the technical support team 
Multiple static IP addresses from BT
Multiple static IP addresses from BT work different to how you might expect. If you have 5 or 13 IP addresses your routers WAN IP will be assigned a ‘peer address’ that will be totally different to the static IP's you have. A ‘peer address’ is essentially a dynamic IP address so depending on what router you use your routers WAN IP (your address on the internet) will change regularly. Not ideal if you want to connect remotely or if you are using a server for emails or remote working.
If you don’t need to connect remotely or if it's not important for your router to always have the same IP address the rest of this will not be relevant (but it might still be interesting). Read on...
Peer IP addresses
A peer IP address is a gateway for your range of static IP addresses. Static IP addresses are NOT normally assigned to devices by the ISP. You will usually set your static IP addresses up yourself, pointing them to the various devices on your network.
It is possible to use the standard router supplied by BT (2700GHV) to assign your multiple static IP addresses to different devices on your network such as a server or webcam. The BT 2700HGV router then (somehow) tells BT what your current peer address is so that you can always connect remotely to your devices.
However, this is a complicated way of doing things. Before the BT2700HGV can assign your static IP’s to devices on your network its DHCP needs to be enabled. This might not be a problem for some people, but if you have a Windows server you definitely don’t want your router to have DHCP enabled because a Windows server likes to be the only DHCP device on a network (if a Windows server detects another DHCP device on the Network it will disable its own DHCP capability).
But all is not lost. There are two simple solutions to make sure that your router always has the same IP address without using a dynamic peer address and without using the routers DHCP.
SOLUTION 1: To get the BT 2700HGV router to always have the same IP address you need to change your IP package with BT from 5 or 13 static IP addresses to just one static IP address. I have not tried this myself, but have been told that if you only have one static IP address the BT 2700HGV will always remember it. This means you can always connect remotely to your router and then create port forwarding rules to connect to the various devices on your network. For those of you who use a Windows server you don’t need to have DHCP enabled on the 2700HGV.
SOLUTION 2): Use a better router like a Netgear. I have 5 static IP addresses with BT and use a Netgear DG834 router (£40 PC World). This router allows you to enter a static IP address in the “Static IP” field so it always has the same WAN IP address on the internet. This makes remote connections easy. A Netgear will also allow you to backup your routers configuration which you have probably spent many hours configuring. The BT 2700HGV has no way of backing up it’s configuration.
How do I know all of this?
The reason I know so much about this is that I’ve been though it all. A while back we had some problems with our BT broadband dropping out. I thought it might have been our Netgear router so I connected the BT 2700HGV router that we got when we ordered the broadband to see if that helped. I tired for ages to get the 2700 to remember my static IP address (like the Netgear had done for 18 months without a problem). I tried everything; I looked on the internet, user forums etc. etc.
After spending 5 hours I eventually rang BT (at 1am!!) who said that their static IP addresses didn’t work like that with the 2700HGV. The advisor actually said that Netgear were better routers. This is when I found out all about peer IP addresses. It was the BT adviser who said that if I wanted to get the BT2700HGV to always have the same static WAN IP address the easiest thing to do was change my IP package from 5 static IP’s to just one static IP. Then the IP would really be ‘static’ and the router would remember it.
In the end I reverted back to the Netgear DG834 v3 router and have just entered one of the static IP addresses into the static IP field in the routers configuration. My router always has the same WAN IP address.
Hope this helps.
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Orthohin 
Perhaps you should try ipconfig/release at CMD then do ipconfig/renew 
I'm kinda confuse,what is the best solution about this issue?Thanks! 
