@ChristMagist 

Hey

It sounds like what you are trying to do is ROAMING, this can happen one of two ways.  Forced Roaming, or Seamless Roaming.

Forced roaming is where the device (phone, tablet etc) loses signal from A and decides to reconnect to B.  This is forced as its got no choice but to do the move, it will also kill any connection you have in place.  It literally drops and reconnects.

Seamless roaming is the opposite, it requires a controller of some type on the network watching the devices.  As it sees the signal on A going down and B going up, the controller will move the device from A to B without disconnecting, hence the name Seamless.  

In short, you cant seamless roam with routers, totally the wrong devices for this.  Also the ISP router is unlikely to to support any controller either.  

If you want forced roaming.. set the ISP router as the DHCP giving out 192.168.0.x addresses.  Set the other routers into AP mode and disable their DHCP server (if it doesnt do it automatically).  They will then pull a DHCP from the ISP router under the 192.168.0.x range.. should be good to go.   

Big disadvantage of this setup, roaming will be awful as client devices hang on to the old AP until grim death.   You will need to set the RSSI threshold on each AP to kick the users at an apprpiate level (say -70db) so as to force them to roam.  Its not elegant and likely what was in place already. 

Honestly.  Routers are not what you want here, you really should only have one router on your network, especially of this grade of hardware.

Better option would be to return the Ax1500 routers and buy APs with a controller, let the controller do the work.  However you wouldnt be able to use your ISP WiFi as it wont support roaming so you will need 3x APs.

Options are..

1.   3x EAP225s with an OC200 Controller.   Around €270 for all 4 devices at a guess.   Turn off the WiFi on the ISP router, connect the controller and plug in the APs.  Configure the lot from the controller.   Perfect solution for a school setup, done it dozens of times!

2.  Cheaper option, 3x pack of Deco devices, something like the M4R.   Set them for AP mode and they will work as one large network under one SSID.  Easy to setup and cheap.    Again disable the ISP WiFi

The EAP option (1.) is the much better option, you will get more control, better management and its a business grade solution.  The Deco is good, but wont be as tidy an option, in a school you should be looking to avoid using "home grade" hardware imho.   The EAP225 and OC200 is a good starting point for you

Hope that helps!

  @Philbert 

Hello!

"If you want forced roaming.. set the ISP router as the DHCP giving out 192.168.0.x addresses.  Set the other routers into AP mode and disable their DHCP server (if it doesnt do it automatically).  They will then pull a DHCP from the ISP router under the 192.168.0.x range.. should be good to go."

First question: This is what happened when i set the AX1500 routers to ap mode. However, they were still three different networks. I set the SSID of router 2&3 to the same as the ISP router and still there were three networks on the network list. How do you make all of them to appear as one network?

Second question: Do i need to turn off the main router when inserting router 2&3?

The setup with the netgear routers works fine, they are a bit old thats it. So it is no problem with using routers.  But the netgear routers dosent use  the 192.168.0.x network. They use DHCP and distribute 192.168.1.2 - 192.168.1.254 ip addresses. So the routers gain an ip address from the main router of the 192.168.0.x range but they distribute another address with another network id.

Third question: How does this happen and still appear as one network in the network list?

  @ChristMagist 

Hello again, hopefully I should be able to answer some of these for you.. :)

First question: This is what happened when I set the AX1500 routers to ap mode. However, they were still three different networks. I set the SSID of router 2&3 to the same as the ISP router and still there were three networks on the network list. How do you make all of them to appear as one network?

When using 3 separate devices like you are currently, its always going to be 3 networks.   These devices are not aware of each other, therefore when Windows looks for SSIDs it will see a reply from each AP that just happen to have the same name / password, they are not talking as 'one network'.   As they are not working together this is always going to be the case sadly.   It all comes back to the Forced vs Seamless roaming, when you have seperate devices broadcasting the same SSID they do so thinking they are the only one with that SSID, which is not the case. 

If there is a controller in place, something like Deco or Omada then the controller replies as one network and not each AP.  Windows will see one reply regardless of 1 / 3 / 50 APs being in range. 

Second question: Do i need to turn off the main router when inserting router 2&3?

No, again the main router wont know or care that the other router is there.  It will just treat it as another device looking a DHCP address. 

The setup with the netgear routers works fine, they are a bit old thats it. So it is no problem with using routers.  But the netgear routers dosent use  the 192.168.0.x network. They use DHCP and distribute 192.168.1.2 - 192.168.1.254 ip addresses. So the routers gain an ip address from the main router of the 192.168.0.x range but they distribute another address with another network id.

Sounds like they are then operating in Router Mode, thats literally what router means, namely it routes data from one network to another.   This is not ideal if honest as you are segmenting your network with routers, nothing wrong with doing that par say.. its just bad practice if honest.   There should really only be one router on the WAN connection, everything internal is Switches or APs. 

The problem with having multiple subnets in play like you are is NAT, you may find you need to port forward some traffic accross the netgear devices.

Third question: How does this happen and still appear as one network in the network list?

They should appear as 3x devices with 3x networks.  It could be that the RSSI or Power Levels are tweaked enough to segment these apart enough that only one replies.   Unless there is some management in there (I dont think there is), it could be a red herring / false positive.