The sim works however you would need to buy a data pack for the global+ zone - probably better to buy the global+ SIM card from the beginning. Details of coverage are available here https://www.qynamic.com/zones/
The starting credit has a validity period of 90 days from activation. Additionally topped up credit has a validity period of 30 days. You can find more information on the manufacturer's website: https://www.qynamic.com/de/produkt/
According to my information, the starting credit expires 90 days after activation. The SIM card itself is valid for 180 days. Additional volume can be purchased in the Qynamic app, which is valid for 30 days. Further information: https://www.qynamic.com/de/produkt/
Whats App works! After installing the Q-Sim and opening WhatsApp, WhatsApp asks if you want to keep the number and then says "Yes" or "Keep old number" I can't remember exactly. But that's how it works :)
Yes, you can. Set your own SIM to calls and SMS and the Q-SIM to data/Internet. In this way, Whatsapp and the like will also run via your own number but the data volume will be used by the Q-SIM.