I ran into the same problem, my iOS device just wouldn’t connect to the Metro server while testing a React Native app. After trying a bunch of things, what finally worked was changing the Wi-Fi Private Address setting on the Mac.

Here's what I did:

1. On the Mac, go to Settings > Wi-Fi.
2. Tap the Details button next to the connected network.
3. Switch Private Wi-Fi Address from Rotating to Fixed.

After this change, my device was able to connect to the Metro server without any problems.