How to fix lost internet on ubuntu 20.04?

Question

Computer networks

The essence of the problem:

It is worth ubuntu 20.04 LTS, after replacing the router from the old one to the new one, there was a problem of lack of Internet 5-10 seconds after the system was started. The provider said that he had no errors and everything was set up correctly, which confirms that the Internet is working via wi-fi and on windows on the same computer. The Internet does not want to work specifically on ubunt. Well, he works for 5 seconds and dies, then 5 minutes pass, he comes to life for another 5 seconds and so on in a circle.
He sees the network equipment, I checked through the lspci command, he shows me the ethernet adapter along with the driver for it. DNS server, it also automatically determines the correct one (Yandex).
Maybe someone faced such a problem and knows the solution?

Answer 1

In short, thank you all for your comments and answers, as well as for trying to help.
After all the manipulations, a banal reinstallation of Ubuntu helped, and, at the same time, updating it from 20.04 to 20.10

Answer 2

through what is the network configured in ubuntu? when the Internet falls off - the router responds?
and the fact that Yandex DNS are issued - did you do it on purpose?