In an elevated Powershell
Get-ItemProperty -Path "HKLM:\Software\Microsoft\Windows\CurrentVersion\Lxss" -Name "NatNetwork"
This prints the current network in CIDR, the /20 is WAY too many hosts for what almost anybody might need.
Set-ItemProperty -Path "HKLM:\Software\Microsoft\Windows\CurrentVersion\Lxss" -Name "NatNetwork" -Value "172.17.128.0/24"