How can I add another WX3100 to a mesh of already 2

Hello,

I have 2 WX3100 ap in a mesh and wanted to add another. However, no matter how I try to do this, via cable or WIFI, the 3rd WX3100 only appears as a separate access point, not part of the web. In the app it is depicted differently. Is it possible to make it part of the mesh?

Also, the identification number seems to be much smaller, so I thought, maybe it is an older version.