I also have exactly same problem. And I think it 's because of some Windows update installed recently.
Similar problems I have over a year ago when Windows 8.1 has been released and Dell support advised me not to install it or rollback to Windows 8, saying that 8.1 is not reliable yet.
And even earlier I also had problems with WiFi and WiFi adapter has been replaced in service center , but it didn't help.
Now Dell support says me that my support plan doesn't include phone support and I should bring my laptop to service center.
I have uninstalled lates Windows update but it do not help.
In general I am a bit tired of Dell service and I see a lot of complaints about WiFi adapter in 5521 (15R) model, but Dell do not care. And definetly my next laptop won't be Dell.