Windows Direct Access and F5 VIP

I have Windows 2012 with Direct Access installed on a stand alone server using a single NIC with static IPv6 and IPv4 addresses. The F5 is passing https to the IPv4 address and all is well, clients can connect externally.

However, when I enable load balancing with another Direct Access server and use the option "external load balancer" clients can no longer connect. The VIP used for load balancing in the F5 is now down (not passing hhts 443). Any ideas? Thanks.