DNS timeout on public IP

Hi, I've just installed a Hyper-V then SBS as guest.
And noticed DNS server becomes non-responsive after
a while, say fifteen minutes. It is related to Remove Access
service. To recover DNS:


I open NAT configuration :
IP v4 > WAN Interface > Services & Ports

I just click any unchecked service, Service Properties
window appears.

I simply hit the Cancel button. Then hit OK for Services & Ports

In a brief; I do NO CHANGE.


Magically, DNS becomes accessible from anywhere.
For another 15 minutes.

I've examined Event viewer, no any clue.

Public IP is always responsive on SBS server.

Any idea? Thanks!