Question about Zyxel WX3100 WLAN Mesh Repeater

Has anyone already got this to work with an XMG3927-B50A (very widespread in Switzerland "thanks" to G.Fast but technically outdated), i.e. Wi-Fi Extender (Gateway > WiFi > Extender) via MPro Mesh? It works with the LAN cable connected, but unfortunately they can't be found via Wi-Fi.