Get all available VPN connections:

Get-VpnConnection

If it’s not there, then get all available VPN connections from the global phonebook:

Get-VpnConnection -AllUserConnection

To add a route: (use -AllUserConnection at the end as needed):

add-VpnConnectionRoute -ConnectionName "My VPN" -DestinationPrefix "192.168.31.0/24"

To verify it:

$conn = get-vpnconnection -ConnectionName "My VPN"
$conn.routes

If you need to remove a route:

Remove-VpnConnectionRoute -ConnectionName "My VPN" -DestinationPrefix "192.168.31.0/24"