Can I use the NinjaTrader Platform on WiFi or what is recommended?

Unfortunately NinjaTrader does not have a have a minimum internet connection speed. There is nothing they suggest.

But they are pretty clear about it, that if you intend to trade live, they do recommend to take the fastest internet connection possible.

While this speed would likely be sufficient for receiving data, They never recommend using a 3G or WiFI connection when using NinjaTrader. Instead, they recommend using a hardwired broadband connection.

The reason for this is that WiFi and 3G connections are far more likely to connection issues and excessive latency. This would be more a matter of the connection's overall health, not it's speed.