As shown in the picture, I will get 3 public ips and hex is the first router connected to the modem.
hex operates a dhcp server of 192.168.1.0/24 so that roaming works by putting AP1 and AP2 on the same network.

Give openwrt router a separate public ip2 to create another network, and openwrt will have a dhcp server of 192.168.2.0/24.

The settop box will be given a public ip3.

Is there any problem with implementing that setting through hex?

I've been looking for a wired router with 5 ports and there are no other devices on the market that are suitable. I had an rb750gr3 for a while a few years ago. At the time, there were no packages such as openvpn udp, dns over https, wireguard, and eventually sold hex and configured openwrt.

Since I am still using packages such as transmission and docker in openwrt, I want to configure as above.

Hello!

I dont have experience with settop box and openwrt and I dont know why you need a public IP on both but I will assume you can assign a static IP as in most network devices.
Maybe you should do the following:
* Create on Mikrotik a new bridge
* Add modem port, openwrt port and settop box port to the new bridge
* Add a static public IP on bridge (Or maybe a DHCP client)
* Config an static public IP on openwrt and settop box (Or maybe a dinamyc IP)
* Add a source nat or masquerade, to all comming from 192.168.1.0/24 and outgoing to the bridge (but 192.168.2.0/24), to be able to reach internet from AP clients
* Add route if you need to reach the 192.168.2.0/24 network and maybe filter rules to allow what you need

You will need that you ISP give you at lease a /29 IP pool.
Regards,
Damián