Hi Attila,
Follow below steps to isolate the issue:
Check the wireless connection in Safe mode with networking (restart the computer and tapping F8 on dell logo. Select Safemode with networking).
Uninstall and reinstall the wireless network drivers for the computer.
Uninstall:
- Open the "Start" menu and click "Control Panel."
- Type "Device Manager" in the search bar at the top right of the screen, then click the "Device Manager" heading that appears in the search results.
- Click the "+" symbol next to your desktop's name, and again next to "Network adapters."
- Right click “Wireless card”, click “Uninstall.
- Check the box “Delete driver software for this device”, click ok and follow the on-screen instructions to uninstall the wireless drivers.
Install the wireless network drivers for the computer.
- Visit the following website: http://dell.to/T2hKYc
- Enter the Service tag of the computer in the box provided and click “Submit”.
- In the next page, click drop down arrow to select the Operating System which is installed in the computer.
- Under category Network, select the drivers accordingly and click “Download File” and save the file on desktop.
- Follow the on-screen instructions to update the Wireless drivers.
If the issue persists, try updating the update the wireless router's firmware to the latest available.
For more information, I have provided a link below which can fix the issue:
Below mentioned link would also help you with the settings for wireless internet:
If the issue persists, run diagnostics on the computer from the below mentioned link:
http://dell.to/U4rPVx
From the above link click on “Component “and select “Network card “and run diagnostics.
Awaiting your response!