To access your OpenWRT admin panel from the internet, first login to your router, go to Administration, then Network, Firewall and open Traffic Control.
Here add a new entry, with WAN as Source and Device as Destination. Save and go to Firewall – Traffic Redirection. Add a new rule, where Zone is WAN, Protocol – TCP, 80 as External port, use your router internal IP which is usually 192.168.1.1, and use the OpenWRT websrver port, which is usually 80, as internal port.
Save all changes and reboot the router.
Now if you connect to the internet from somewhere else, like your 4G phone and enter your router external IP in your browser, you’ll be able to connect to OpenWRT admin panel. Please note, that your router must have at least dynamic, but own external IP to be able to do that. If the IP is dynamic, it’s a good idea to install a dynamic dns client on to the router, so you could connect to it even if you don’t know current external Ip address of the router. If your router is behind a single IP as well as all other customers of your ISP, than try to ask the provider to open certain ports to your router and change the port forwarding rule accordingly.
Now, if you enter your ISP IP address and the given port or the dynamic dns and the port, you’ll be able to connect to your OpenWRT router from the internet.