Dell XPS Wireless Problem, Unable to detect Wifi

Hi all,
Currently I'm using Dell XPS M1330, a model few years back. These few days I have problem to connect to my Unifi Wifi. Situation as below:
1. Iphone 4: Connect to Wifi no problem at all
2. Dell XPS M1330: Can detect Wifi from other houses but except mine. Even I bring near to router also same.
3. Unifi router: Turn off and on multiple times don't solve the problem. Iphone 4 keep on able to connect, but not Dell.
4. Dell direct LAN cable, can online.

If router is having problem, why Iphone 4 can online all the time?
If Dell wireless is having problem, why Dell can detect all the wireless except mine?

May I know what could be the problem? Thanks!