If you are employed in multiple EU countries or run your business outside of the Czech Republic in some EU country, EU social security coordination rules apply to you. In order to find out under which country's legislation you are covered according to EU rules, you need to submit an application for A1 form in the country of your residence. The application should be submitted at the appropriate social security institution, ideally before you start working at MENDELU.
Principles of EU social security coordination (together with a directory of the relevant contact points in each country).