When your ISP implements Carrier-Grade NAT (CGNAT), traditional port forwarding becomes impossible because:

1. No public IPv4 address assigned to your router
2. ISP's NAT layer blocks all unsolicited inbound traffic
3. Dynamic IP changes make static configurations unreliable

Here are technical solutions I've implemented successfully:

1. Reverse SSH Tunneling

Setup a cheap VPS as relay point:

# On your local machine behind NAT:
autossh -M 0 -N -R 2222:localhost:22 user@yourvps.com

# On VPS (after connecting):
ssh -p 2222 localhost

2. WireGuard with Persistent Keepalive

Configure outgoing connection from NAT-ed network:

# Local WireGuard config (initiates connection)
[Interface]
PrivateKey = local_private_key
Address = 10.0.0.2/24

[Peer]
PublicKey = vps_public_key
Endpoint = yourvps.com:51820
AllowedIPs = 0.0.0.0/0
PersistentKeepalive = 25

3. Cloudflare Tunnel (No VPS Required)

Modern alternative using Cloudflare's free tier:

# Install cloudflared
curl -L https://github.com/cloudflare/cloudflared/releases/latest/download/cloudflared-linux-amd64 -o cloudflared

# Authenticate
./cloudflared tunnel login

# Create tunnel
./cloudflared tunnel create vpn-tunnel

# Route traffic
./cloudflared tunnel route dns vpn-tunnel vpn.yourdomain.com

For pfSense/OPNsense users, implement a hybrid solution:

1. Set up OpenVPN client on firewall (outgoing)
2. Configure port sharing on VPS:
   iptables -t nat -A PREROUTING -p tcp --dport 1194 -j DNAT --to-destination [local_vpn_ip]
3. Enable VPN kill switch to maintain connectivity

Essential cron jobs for reliability:

# Check tunnel status every 5 minutes
*/5 * * * * ping -c1 yourvps.com || systemctl restart wireguard

# Dynamic DNS update (if you get occasional public IP)
*/15 * * * * curl -X PUT "https://api.cloudflare.com/client/v4/zones/[ZONE_ID]/dns_records/[RECORD_ID]" \
-H "Authorization: Bearer [API_TOKEN]" \
-H "Content-Type: application/json" \
--data '{"type":"A","name":"vpn.yourdomain.com","content":"$(curl -s ifconfig.me)","ttl":120}'

When your ISP uses Carrier-Grade NAT (CGNAT) - common with 4G/LTE networks - traditional port forwarding becomes impossible. Your VPN server sits behind multiple NAT layers without a public IP.

1. Reverse SSH Tunneling

This creates a persistent tunnel from your local network to a publicly accessible server:

# On your local machine (behind NAT):
ssh -R 2222:localhost:22 -N user@your-vps-ip
# Then connect to your VPS's port 2222 to reach local VPN

2. WireGuard with Persistent Keepalive

Configure WireGuard to maintain an outbound connection:

# /etc/wireguard/wg0.conf on local server
[Interface]
PrivateKey = your_private_key
ListenPort = 51820
PersistentKeepalive = 25

[Peer]
PublicKey = remote_peer_key
AllowedIPs = 0.0.0.0/0
Endpoint = your-vps-ip:51820

3. Cloudflare Tunnel (No VPS Required)

Free alternative using Cloudflare's infrastructure:

# Install cloudflared
wget https://github.com/cloudflare/cloudflared/releases/latest/download/cloudflared-linux-amd64
chmod +x cloudflared

# Create tunnel
./cloudflared tunnel create vpn-tunnel

# Route traffic
./cloudflared tunnel route dns vpn-tunnel vpn.yourdomain.com

# Start proxy
./cloudflared tunnel run --url http://localhost:1194/ vpn-tunnel

For protocols like OpenVPN that use UDP, implement hole punching:

# Python example (simplified)
import socket

# Both client and server send packets simultaneously
def punch_hole(public_ip, port):
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    sock.bind(('0.0.0.0', port))
    sock.sendto(b'PUNCH', (public_ip, port))