1. GST cannot be claimed for personal/home connections. They have separate commercial plans if you need GST bills to claim tax
2. When buying new connection, you need to pay extra 1000 only for STB. They provide 1 month free OTT apps. After that you need to recharge for higher plan which has OTT subscription, else 399 plan does not have OTT access.
3. That is totally upto you.
4. I think STB works on both wifi and LAN cable (one is provided in the box)
5. Currently only a single model is available which has both 2GHz and 5GHz wifi bands