Hi Sebastian,
DX100's wifi sucks. It just works on/under/next to modem/router. But there can be also problem about your device, so don't hesitate to contact with iBasso. It's not important to buy from them, iBasso gives 10 years free labor support.
Also at first, you can contact via Paul from iBasso, he's very active on HF.
Here is Paul's profile: http://www.head-fi.org/u/415799/Paul-iBasso