I solved my issues with following steps:
1. Restart router/modem
2. Open Command Prompt (windows search > type cmd > right click and "Run as administrator")
3. In cmd type:
ipconfig /flushdns > and click enter
ipconfig /release > click enter again
ipconfig /renew > wait a while, then...
4. Restart PC

Yesterday I was able to play whole day without a single disconnection.