Why can't I connect to my home router via a 4G modem anymore, even though it used to work?

YOUR MOBILE OPERATOR IS HIDING YOUR MODEM BEHIND A CGNAT (CARRIER-GRADE NAT) / PRIVATE IP ADDRESS.

With traditional cable internet, your provider gave you a public IP address, which made setting up DDNS easy. On a 4G cellular network, however, all incoming inbound connections from the outside are completely blocked by the mobile carrier.

The Mobile Network Trap

Cellular towers use CGNAT technology to save IP addresses: thousands of subscribers share a single external IP. Your modem receives an isolated internal address within the network, meaning any attempts to reach it from the internet via DDNS or classic OpenVPN hit a brick wall at the carrier's firewall.

How to Get Access to Your Modem

Request a dedicated static public (white) IP address from your mobile operator
Set up your modem to automatically connect to an external rented proxy server/relay
Use modern overlay utilities like Tailscale or ZeroTier to bypass these limitations