You need to assign one of the /64 from HE statically to your LAN as an additional virtual address. To be able to use that /64 for devices to access the Internet you need to set up a tunnel according to HE's documentation.
But why did you do this in the first place? I though you wanted to use static ULAs in addition to your ISPs GUA. Now you got another set of GUAs - a whole /48 of them - from HE.
I then manually set the IPv6 of that machine as static, and assigned an IP from the ULA I'd just created. I can't make it connect to the internet via IPv6, but I guess that's expected because ULAs aren't supposed to be routable...? If it's supposed to work, then I have to figure out the gateway to u
I initially planned to use static ULAs, but decided to move to static GUAs because I found that there are some idiosyncrasies with AdGuard Home.