@MiroM 

No, all AP are with static addresses

  @Vincent-TP 

1. Firmware Updates

Check if there are any firmware updates available for the APs, the switch, and the router. All devices are with a latest firmware

2. Power and Cable Checks

• Although you mentioned that the switch ports show as up and 1G connectivity, it's still possible that there are intermittent cable issues. Check the physical connections of the APs to the switch. Look for any signs of cable damage, such as frayed wires or bent connectors.All cables are new CAT6a and there are no any  cable damages.
• Consider swapping out the cables for a few of the APs that are frequently disconnecting to see if the problem persists. Also, check the power sources of the APs. Fluctuations in power can cause the APs to reboot or lose connection. If possible, use a power quality tester to check the electrical outlets that the APs are plugged into. All APs are using PoE from the switch

3. VLAN and Network Configuration

• Since the network is segmented into 3 VLANs, there could be an issue with the VLAN configuration. Double - check the VLAN settings on the switch and make sure that the APs are correctly configured to communicate with the appropriate VLANs.All APs are correctly configured to communicate with the appropriate VLANs
• There might be a misconfiguration in the PPSK settings without RADIUS that is causing the APs to have trouble authenticating or communicating with the controller. Review the security and authentication settings for the WiFi network. The configuration of PPSK settings without RADIUS are the same on all sites.

4. Network Traffic and Bandwidth

• In a school environment, there can be a lot of network traffic, especially during peak hours. Check the network utilization on the switch and the router to see if the APs are being overwhelmed with traffic. If the bandwidth is maxed out, it could cause the APs to lose connection or become unresponsive. The network traffic is less than 30% of ISP load

5. Interference

• Check for sources of interference that could be affecting the APs. In a school building, there could be other wireless devices, such as cordless phones, microwave ovens, or other wireless networks that operate in the same frequency range. The channel utilization is below 15%
• Use a wireless spectrum analyzer to scan for any sources of interference and try to adjust the channel settings of the APs to avoid the interference.  
•  
• How to optimize wireless performance of EAP products

6. DHCP and IP Addressing

• Make sure that the DHCP server (which could be the router) is properly configured and has enough available IP addresses for the APs and the clients. If the APs are losing their IP addresses or having trouble obtaining a valid IP, it can cause them to disconnect from the controller.  The DHCP server (which is the router) is properly configured and has enough available IP addresses for the clients. All APs are using static IP addresses.
• Also, look for any IP address conflicts in the network that could be causing problems. No any IP address conflicts

@MiroM, some more followup questions...

• Are the APs operable at all when disconected?  Doe it appear that it's just the management traffic that's impacted or everything?
• Has it happened to all APs?  Are some disconecting more often than others?
• Is this happening when the building is unoccupied?
• I think with 18 APs you should be ok on your POE budget, but does it look sufficient?  Are there any POE devices plugged into the APs?

  @D-C

• Are the APs operable at all when disconected?  Doe it appear that it's just the management traffic that's impacted or everything? All client are disconected from these APs, no ping to management address, so all trafic is impacted, not only management.
• Has it happened to all APs?  Are some disconecting more often than others? It happened to almost all APs
• Is this happening when the building is unoccupied? No, it's happening only when there a lot of clients.
• I think with 18 APs you should be ok on your POE budget, but does it look sufficient?  Are there any POE devices plugged into the APs? Only 30 % of PoE budget of the switch is using.

@MiroM, you mentioned almost all APs have had this issue, so can you swap an AP that's never had the problem with one that consistantly has the problem to see what happens? If your able, maybe swap with one from a school that doesn't have the problem at all.  I'm wondering if it could be some unforseen environmental issue like people placing bags, coats, etc. in front of the AP preventing heat dissipation.  Beyond that, maybe swap the POE switch if possible or anything else that can help isolate the problem.

I forgot to ask earlier, but do the APs actually reboot/restart (uptime reset) or just unresponsive for a period of time?

  @D-C 

Hello,

I manage this network's remotely, so i'm not on site and cannot do physical changes. All APs are mounted on 2,5 height meters from the floor, so no chance to place anything.

The APs actually are unresponsive at all, and only way to back them connected again is to do PoE recovery on the switch port

@MiroM, because it's a school I was worried that the kids could be doing something, but not if it's installed that high.  Remote troubleshooting can be challanging; I can only think of a few more things to check remotely.

You have more APs deployed, so is it correct to say that the client to AP ratio at the problem site is similiar to a working site?  Is there any kind of client spike before going unresponsive?

Have you used the CLI to run the cable diagnostics on the working and problem APs?  I assume your switch supports this, so curious to see if there's any correlation between the problem frequency and cable length or if anything else interesting shows up.