@VolkerS 

What type of switch do you have?

  @MR.S 

ER7412->SG2428P->EAP115Wall
This is the path to the client.

This phenomenon occurs frequently. Eventually, an IP address will be assigned.

This is probably the cause of the unstable network.

  @VolkerS 

yes that was strange, SG2428P should show you the ip address of the clients, i have such a switch myself. have you tried restarting everything, switch access points and controllers.

you can also try and upgrade the firmware if you haven't done so.

  @VolkerS 

Sometimes the IP will disappear from the client list if the device is very very quiet and had long periords of inactivity.

If you enable ARP Scanning on the network in question, it should resolve this

  @GRL

Hello, where can I enable ARP?
I can't find anything about it.

  @VolkerS 

enable ARP Detection on network interface, adavanced settings

  @MR.S 

Thank you for your suggestions.
All devices have been restarted.
Since the system has been producing unstable connections (Wi-Fi) from the start, I always keep the firmware up to date, hoping that something will improve. Maybe the problem is indeed with the router.
"The internet" warns against TP-Link routers, especially the 7 series.
I have been using the Omada controller for several years without any problems at multiple locations.
However, always with a Fritzbox as the router and an unsegmented network.

  @MR.S 

I can't find this option. I'm too stupid.

  @VolkerS 

It's not certain that you have it, do you have the latest version on your controller and router? Try and upgrade your router to the latest version first