Hi Cyril
Currently, these relationships are not supported in Native. You can look at adding an entity that makes *-* into two 1-*
More information on that here
https://fmhelp.filemaker.com/help/18/fmp/en/index.html#page/FMP_Help/many-to-many-relationships.html
Also, please see this forum post: https://forum.mendix.com/link/questions/97668
Mendix are looking to add the feature this year