Currently you can only use a fixed amount of RAM because setting the VM to dynamic then Hyper-V is unable to give the machine extra RAM as needed.
E.g. set the VM to 1GB with dynamic RAM at min: 512MB max: 4GB, the demand becomes around 3GB but the actual use remains at 1GB.
I have a Fedora Server VM running on the same host and these settings work perfectly fine with it’s RAM usage going up and down as needed.