You're so wonderful!
After putting the sim into my phone and turn on data roaming, I have used the USSD code to top up to total euro 25 just now. After that use *101# to check the balance is now euro 25. But in the service portal, it still shows the balance is euro 10, I guess it may take a few hours to update from the mobile network just like last night after I activated the sim card the portal shown the balance is zero but this morning it shown the balance is euro 10.
Now it seems perfect that I just need to subscribe the 15 euro monthly data plan just a day before my departure from home.
I recall someone in the forum reminded that it is better to change the data plan back to usage base before leaving Germany to save the money.
Anyway, you gave me a big help indeed!!
Cheers