1 - Activate mimugmail's community repository

2 - Install AdGuardHome from System --> Firmware --> Plugins

3 - Activate and start AdGuardHome from Services --> AdGuardHome

4 - Opnsense - System - Settings -General

      Untick: Do not use the local DNS service as a nameserver for this system
      Untick: Allow DNS server list to be overridden by DHCP/PPP on WAN

5 - Opnsense - Services - Unbound - Dns Over Tls

      Set the desired dns servers, ej, Cloudflare:
      Server IP: 1.1.1.1
      Server Port: 853
      Verify CN: cloudflare-dns.com

6 - Opnsense - Services - Unbound - General

     Listen Port: 5353

7 - Navigate to http://your.opnsense:3000/ ( 192.168.1.1:3000 ) to complete the setup Adguard

8 - Adguard Home - DNS Configuration - Upstream Servers: Add router_ip:5353  ( 192.168.1.1:5353 ) Delete those that exist

Security Extra: https://www.sunnyvalley.io/docs/network-security-tutorials/how-to-configure-opnsense-firewall-rules#1-allowing-only-specific-dns-servers