How to set up a DMZ in this case?

Hello redditors, finally I got fiber and a public static IP in my house. Now I need some servers but due to security reasons I need first a DMZ.

This is my situation: https://imgur.com/a/vtotZJd (the DMZ is not yet configured)

Basically the ISP Router is useless, I can only forward ports, assign static IP addresses and little more. Than I have and old fully configurable router running OpenWRT.

I know in a proper DMZ my servers should be in the middle, between internet and my LAN but unfortunately this is the only way to go I can think about.

How should I configure my firewall and a subnet in order to be the most secure as possible?