If you say that at the beginning you have had problems connecting, and now it does not work on Android or Windows, it is clear that your problem is the network card. It may just not make good contact or it may be broken.|
Only two possible solutions:
1. Replace the network card (with the help of a technician)
2. Acquire a compatible USB Wifi adapter for Android and Windows and use the tablet with that adapter.
If the tablet is under warranty, you can open a ticket in the seller indicating the fault and requesting repair or exchange.