To setup a custom DNS on your TIM ZTE H388X router:

• head to "Rete Locale" -> "LAN" -> Server DHCP
• execute this script in the browser dev tools console

document.querySelectorAll('#template_DHCPBasicCfg [style*="display: none;"]').forEach(el => {
    el.style.display = ''; // Removes inline display:none
});

• you should see