Since I have installed the os there is no possibility to connect using the WiFi. I am still able to connect to Internet using the USB thetering of my phone.
Would you please advice on how to get back online using WiFi?
Sounds like you are missing a driver. I had a similar problem and was fixed when I ran updates after install. If not you need to find you wifi card and download the driver for it. What are the specs for your computer?
Maybe you have some drivers or packages missing. This worked for me: WiFi icon on wing panel disappeared. Try it. I face such an issue many times (mainly after huge updates) and, until it is fixed, we will experience that problem.