telekom.js

const apiUrl = "https://pass.telekom.de/api/service/generic/v1/status"

let widget = await createWidget()
widget.backgroundColor = new Color("#777777")
if (!config.runsInWidget) {
  await widget.presentSmall()
}

Script.setWidget(widget)
Script.complete()

async function createWidget(items) {
  let fm = FileManager.local()
  let dir = fm.documentsDirectory()
  let path = fm.joinPath(dir, "scriptable-telekom.json")

  const list = new ListWidget()
  list.addSpacer(16)

  try {
    let r = new Request(apiUrl)
    // API only answers for mobile Safari
    r.headers = {
      "User-Agent": "Mozilla/5.0 (iPhone; CPU iPhone OS 13_5_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1.1 Mobile/15E148 Safari/604.1"
    }
    
    let data, fresh = 0
    try {
      // Fetch data from pass.telekom.de
      data = await r.loadJSON()
      // Write JSON to iCloud file
      fm.writeString(path, JSON.stringify(data, null, 2))
      fresh = 1
    } catch (err) {
      // Read data from iCloud file
      data = JSON.parse(fm.readString(path), null)
      if (!data || !data.usedPercentage) {
        const errorList = new ListWidget()
        errorList.addText("Please disable WiFi for initial execution.")
        return errorList
      }
    }
    
    const line1 = list.addText("Telekom")
    line1.font = Font.mediumSystemFont(12)

    const line2 = list.addText(data.usedPercentage + "%")
    line2.font = Font.boldSystemFont(36)
    line2.textColor = Color.green()
    if (data.usedPercentage >= 75) {
      line2.textColor = Color.orange()
    } else if (data.usedPercentage >= 90) {
      line2.textColor = Color.red()
    }
  
    const line3 = list.addText(data.usedVolumeStr + " / " + data.initialVolumeStr)
    line3.font = Font.mediumSystemFont(12)
    
    list.addSpacer(16)
    
    let line4, line5
    if (data.remainingTimeStr) {
      line4 = list.addText("Remaining time:")
      line4.font = Font.mediumSystemFont(12)

      line5 = list.addText(data.remainingTimeStr)
      line5.font = Font.mediumSystemFont(12)
    }
    
    // Gray out if local data instead of Telekom API data:
    if (fresh == 0) {
      line1.textColor = Color.darkGray()
      line2.textColor = Color.darkGray()
      line3.textColor = Color.darkGray()
      if (data.remainingTimeStr) {
        line4.textColor = Color.darkGray()
        line5.textColor = Color.darkGray()
      }
    }
    
  } catch(err) {
    list.addText("Error fetching JSON from https://pass.telekom.de/api/service/generic/v1/status")
  }

  // Add time of last widget refresh:
  list.addSpacer(4)
  const now = new Date();
  const timeLabel = list.addDate(now)
  timeLabel.font = Font.mediumSystemFont(10)
  timeLabel.centerAlignText()
  timeLabel.applyTimeStyle()
  timeLabel.textColor = Color.darkGray()
  
  return list
}

I don't have time to fiddle around with this, but here's a version that should fix a few of the bugs: https://gist.github.com/Sillium/235dc9fd873b25e321b3299da64c9c38

Please delete the folder iCloud Drive --> Scriptable --> <script name> depending on how you named the script in your Scriptable app.

I also wrote my own version if somebody is interested. https://gist.github.com/wolflu05/05ee63670e095fbe21420645f4b31d5a However, there is a little thing which im currently trying out. I plan to have an indicator of how much data I can consume this day, so I have the same data also for the next days.

Funktioniert auch mit CONGSTAR, da ja im gleichen D-Netz