So you tried this I presume:
In Services, look for "Avira.ServiceHost". If the status is blank or does not say "Running" Open the service and click "Start".
Some users seems to be experiencing this bug even after several answers from Avira.. (https://answers.avira.com/en/question/no-more-avira-icon-in-my-system-tray-34684/page-5).
In my opinion, it comes from Avira and they need to...