Setting up test Hyper-v domain - basic network config

I am trying to set up a mini domain using hyper-v guests purely for testing/learning purposes.
I have  Server 2019 & a Windows 10 pro as VMs, currently both on a hyper-v private switch
Hyper-v is running just on Windows 10 on my home PC

I can't get the two guests to communicate at all. The Windows 10 VM does not get an ip address from DHCP on the server.
With manual IPs they can't pin each other

I have DHCP running on my physical router for my home office network, and have created a scope and activated it.

I believe that guests on a private switch should be able to communicate. I must be doing something very stupid that's wrong, but I can't see what it is.

Thanks for any tips! I'm feeling a bit stupid.. I must be missing something obvious.