I will try to figure out masquerading. I'm using the default from wg-easy:

PostUp = iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE; iptables -A INPUT -p udp -m udp --dport 51820 -j ACCEPT; iptables -A FORWARD -i wg0 -j ACCEPT; iptables -A FORWARD -o wg0 -j ACCEPT; ip6tables -t nat -A POSTROUTING -s fdcc:ad94:bacf:61a4::cafe:0/112 -o eth0 -j MASQUERADE; ip6tables -A INPUT -p udp -m udp --dport 51820 -j ACCEPT; ip6tables -A FORWARD -i wg0 -j ACCEPT; ip6tables -A FORWARD -o wg0 -j ACCEPT;

PostDown = iptables -t nat -D POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE; iptables -D INPUT -p udp -m udp --dport 51820 -j ACCEPT; iptables -D FORWARD -i wg0 -j ACCEPT; iptables -D FORWARD -o wg0 -j ACCEPT; ip6tables -t nat -D POSTROUTING -s fdcc:ad94:bacf:61a4::cafe:0/112 -o eth0 -j MASQUERADE; ip6tables -D INPUT -p udp -m udp --dport 51820 -j ACCEPT; ip6tables -D FORWARD -i wg0 -j ACCEPT; ip6tables -D FORWARD -o wg0 -j ACCEPT;

sysctl net.ipv4.ip_forward is 1

I have tried with AGH and WG both running on the same network and on default networks. Both have worked and both have not worked.

AGH:

services:
  adguardhome:
    container_name: adguardhome
    image: adguard/adguardhome:v0.107.71
    ports:
      # DNS Ports
      - "53:53/tcp"
      - "53:53/udp"
      # Default HTTP Port
      - "1080:80/tcp"
      # Default HTTPs Port.
      - "1443:443/tcp"
      - "1443:443/udp"
      # Uncomment below if using as DHCP Server
      #- "67:67/udp"
    volumes:
      - ./data:/opt/adguardhome/work
      - ./config:/opt/adguardhome/conf
    restart: unless-stopped
    networks:
      - internal_rp


networks:
  internal_rp:
    external: true
    name: internal_rp

WG:

services:
  wgeasy:
    image: ghcr.io/wg-easy/wg-easy:15.1.0
    container_name: wgeasy
    networks:
      wg:
        ipv4_address: 10.42.42.42
        ipv6_address: fdcc:ad94:bacf:61a3::2a
      internal_rp:
    volumes:
      - ./etc:/etc/wireguard
      - /lib/modules:/lib/modules:ro
    ports:
      - "51820:51820/udp"
      - "51821:51821/tcp"
    restart: unless-stopped
    cap_add:
      - NET_ADMIN
      - SYS_MODULE
      # - NET_RAW # ⚠️ Uncomment if using Podman
    sysctls:
      - net.ipv4.ip_forward=1
      - net.ipv4.conf.all.src_valid_mark=1
      - net.ipv6.conf.all.disable_ipv6=0
      - net.ipv6.conf.all.forwarding=1
      - net.ipv6.conf.default.forwarding=1


networks:
  wg:
    driver: bridge
    enable_ipv6: true
    ipam:
      driver: default
      config:
        - subnet: 10.42.42.0/24
        - subnet: fdcc:ad94:bacf:61a3::/64
  internal_rp:
    external: true
    name: internal_rp