Quick setup without using 192.168.88.1

Hello!
I have a hEX that has the address 192.186.88.1 (standard setup, NAT enable, default firewall rules), which is connected to the ISP (pppoe) and works correctly (devices connected by wire have access to the internet).

I also have a hAP ax3, if I use Quick set with the address 192.168.88.1 (wan settings from hEX’s DHCP, default mask, DHCP, NAT and firewall) and connect hAP’s wan port to hEX’s lan port everything works correctly (Wi-Fi devices have internet).

However, if I configure hAP ax3 using 10.0.0.1 (DHCP pool with same adress) as a local address (instead of 192.168.88.1) devices connected to hAP ax3 do not have access to the internet.

Please tell me why this behavior occurs?

could be any of a number of reasons. There is an issue I am aware of from version 6.x of routerOS relating to DNS - from my notes

On initial set up, the DNS server handed out by Mikrotik remains at 192.168.88.1 even after the unit IP address is set. This is due to the DNS address remaining set under [DHCP Server → Networks → ]

I suggest that you try to access a known website by IP address. If that succeeds, then look at the issue I have suggested.

What I suggest is you configure the router from a safe spot to make subnet changes and later if you use vlans.

Take etherX like ether5 OFF the bridge in /interface bridge ports

So it looks like
/interface ethernet
set [ find default-name=ether5 ] name=OffBridge5

/ip address
add address=192.168.77.1/30 interface=OffBridge5 network=192.168.77.0

/interface list
add interface=OffBridge5 list=LAN

Now plug in your laptop to ether5, change your IPV4 settings to 192.168.77**.2** and via winbox access as per usual via username password.
Now you should have access and can independently change iP addresses subnets vlans etc without getting into trouble.