PowerShell question - webrequest or curl doesn't work with tasks - windows

Question

I've written (partly by myself) a script which get the ip address with a webrequest and save it as variable for a dyndns update. If I start the script via ISE or directly with PowerShell it works as expected. But if I start it via scheduled task the web request, whether with invoke-webrequest or curl doesn't works. The script will be started but the variable is empty. If I try to save the output of the command to a file its also empty, so I don't know why it doesn't work.
Here is the script:
# Get IPv4 and IPv6 from wtfismyip.com
$strIPv6 = Invoke-WebRequest -uri http://[2a01:4f9:4b:4c8f::2]/text | select Content -ExpandProperty Content
$strIPv4 = Invoke-WebRequest -uri http://95.217.228.176/text | select Content -ExpandProperty Content
$strIPs = $strIPv4 + "," + $strIPv6
$strIPs = [string]::join("",($strIPs.Split("`n")))
echo "IPs" >> C:\temp\test.txt
echo $strIPs >> C:\temp\test.txt
$strDYNDNS_URL = "https://dyndns.strato.com/nic/update?hostname=sub.domain.de&myip=$strIPs"
# Debug Informations
write "IPv4: $strIPv4"
write "IPv6: $strIPv6"
write "Strato URL: $strDYNDNS_URL"
# Strato DynDNS Update
$user = "domainuser"
$pass = "pass"
$pair = "${user}:${pass}"
$bytes = [System.Text.Encoding]::ASCII.GetBytes($pair)
$base64 = [System.Convert]::ToBase64String($bytes)
$basicAuthValue = "Basic $base64"
$headers = #{ Authorization = $basicAuthValue }
Invoke-WebRequest -uri $strDYNDNS_URL -Headers $headers
The scheduled task runs for testing with an admin user with the following settings:
Start with highest privileges
Trigger: every hour
Cmd: powershell.exe ExecutionPolicy Bypass -Command "C:\Daten\Scripte\StratoDynDNS-Update.ps1"
Does have anyone an idea why it doesn't work? Or is there a better solution?
The goal is to make a dns update at strato because the ISP of my client doesn't provide static ips, even with a business internet connection.