To avoid such a problem, change Address Type from Smart IP to Static IP and set each of them a proper IP address which is in the same subnet as the main network.

That might be one guess. How about fixing the bug?

If the accesspoint can't work properly in DHCP mode, DHCP mode should not be sold. I even suspect that this would not solve the problem. It's lke saying: If Word does not work, write your letters in Excel..

The SSID would also change - so are we expected to only use the predefined SSI and password? You can't be serious.

Did you try what I said?
If you select the type of Smart IP (DHCP) and there is a DHCP server forward, the DHCP server in this device will not start up. Otherwise, DHCP server will start up.

The problem is:

When I switch it to fixed IP I will not be able to notice all devices if I swap the main router, especially when I do that from remote.

It could be that the main router wants to assign different gatewas to different accesspoints or maybe deliver other DNS server addresses to the APs. When using fixed IPs, I have to update every single device with every change in the topology.

DHCP has been invented to solve this problem. It might happen that the gateway disappears (viewn from the AP) - maybe a user unplugged a cable temporarily? The internet connection drops for an hour? What I do NOT want then is that the device turns on its DHCP server. Why should it ever think it is responsible for delivering internet to anyone? That's what a DHCP service is for.

And by the way: What is "Smart IP" good for, if you say I should switch to fixed IP?