It's quite likely your packets are now able to reach the destination, but the response packets do not come back (you can check with tcpdump to investigate further). Assuming that's the case, my next question is if you have a forwading-type VS for the Internal network segment? If your nodes are in 192.168.255.0/24, you will need to create a forwarding-type VS for this subnet, and make it listen on the External VLAN. Additionally, the device which acts as a gateway for F5 on the External VLAN must have a destination route 192.168.255.0/24 which points to the floating external IP.
Hope this will help you out.