The mobile phone connected to the DJI drone shows that it cannot join the network.

Your mobile phone says "No Internet access". You can refer to the following:

This is a logo that only appears in the iOS system. After the Wifi connection, it prompts "Unable to access the Internet", and the WiFi icon has an exclamation point. You are prompted to connect to a network (LAN) that cannot access the Internet. This is a normal phenomenon and does not affect your normal use.