You may have issues installing RSAT on Windows 10 1809 if you are using WSUS. The following block of code will address that by setting the WSUS key temporarily, installing RSAT, and then setting the key back to original

Win10_1809_RSAT

# Set variables
$registryPath = "HKLM:\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\AU\"
$name = "UseWUServer"
$value = "0"
$service = "wuauserv"

# Reset variables
$resetValue = Get-ItemPropertyValue -Name $name -Path $registryPath

# Set reg value and restart service
Set-ItemProperty -Path $registryPath -Name $name -Value $value -Force | Out-Null
Restart-Service -InputObject $service -Force

# Install RSAT
Get-WindowsCapability -Online | ? Name -like 'RSAT*'| where {$_.State -eq 'NotPresent'} | foreach {Add-WindowsCapability -Online -Name $_.Name}

# Reset reg value and restart service
Set-ItemProperty -Path $registryPath -Name $name -Value $resetValue -Force | Out-Null
Restart-Service -InputObject $service -Force