Hi willem61,

Welcome to Dell community.

In response to your query, you might first check the device manager for any bangs meaning if the network drivers have not been installed properly, if you find any bangs which usually show with an exclamatory sign in the device manager window you can probably update the network drivers & check for the issue which will be our first step of isolating the issue. I have also provided you  the link for the drivers you need to install on your system. Login to the website & select the respective operating system & select the category network which gives you a  the driver to be installed as you are not able to go online on this system you can download the driver on a different system & copy it to a storage device like a flash drive & install the same on this computer.

Note Drivers are system specific I have provided the appropriate link for the system which you have mentioned Dimension C521

Let us know if this helps

Thanks & Regards

Dell_Dinesh_M

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