@Nikolas01 

If you have the Guest WiFi enabled (client isolation enabled) and a user connects to this WiFi, then ACLs will stop them from getting access to any other clients; this includes your servers.

However this would be the most basic of filtering / control and really should not be used alone.   If you have security concerns at all you should be implementing a full configuration with Gateway, Switch and EAP controls all managed centrally.   On top of that solid AV / LanGuard protection on your servers. 

Is it tru?Can you confirm it? At what level is the internal LAN secured against attacks from the Guest wi-fi interface?

At a basic level, this will achieve what you are after.   However I would not class this as a "secure" solution, its the very least you should be doing and should be implemented as part of a complete solution.