Hi Aha_tom,
As you mentioned "The 2 sites are in different subnet" and "Obviously can’t use Windows NLB for that", yes, you are right. The Windows NLB does not support multiple subnets, and currently we do not have any officially supported build-in NLB solutions for this scenario.
In this situation, I'd like to suggest you get more information and help about Official NLB configuration for this scenario in the TechNet Windows Server forum.
Additionally, if you want to deploy third party relevant NLB solution, we also welcome other community members to share experiences and solutions. I am sorry for any inconvenience you encountered and thank you for your understanding.
Best Regards,
Bourne Zhang