@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...

@VolkerS try upgrading the controller and router to the latest version, It is a relatively new feature

@AndyBH that was strange, with the internet settings you have, you should only use the backup wan when the primary wan is down. so there is actually no need for policy route either. try running...

@WalpicarBR you definitely have a problem with the cable, check the terminations at both ends of the cable, if they are OK then you need to change the cable. if the access point is meshing then you...

@VolkerS try and enable ARP detection on network interface to see if it helps

@AndyBH set up a policy route and direct traffic to the WAN port you want to use as default and enable Use the other WAN port if the current one is down