One thing I’ve seen a few times is that: when you are on your own WiFi network, some routers block the access to its external IP. So you should test it by switching your phone from WiFi to cell connection, and try again. Basically, the idea is that if you are on your WiFi network, you should access it using the internal IP. The external IP is only allowed when you are outside of your home network. At least I know some routers have this restriction.