NLB with IIS on an external infrastructure

Hello,

I set up a 2 IIS with NLB infrastructure. It want it to dispatch my clients 50-50. It works in my internal network but it doesn't work from external when my clients are behind a unique public IP adress (all my connections are directed on my first IIS).

Is there a way to bypass this issue with the Network Load Balancing from Microsoft?
I tried different settings with the Filtering Mode but I can't find the good one..

OS : Windows Server 2012 R2 Standard

Thank you for your help,
Genialo