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

- set your preferred DNS
- save
Ottimo quello che stavo cercando.