It's something that you don't have to worry about. Router probably refreshes itself and your ISP has to assign you a new WAN IP address.
Try 380.62 beta, it implements workarounds for ISPs with broken DHCP servers.
Welcome To SNBForums
SNBForums is a community for anyone who wants to learn about or discuss the latest in wireless routers, network storage and the ins and outs of building and maintaining a small network.
If you'd like to post a question, simply register and have at it!
While you're at it, please check out SmallNetBuilder for product reviews and our famous Router Charts, Ranker and plenty more!