Hi Nadav,
The fact that it gets stuck on "securing gateway" and doesn't even trigger the Authenticator app is a big clue. This usually points to a handshake issue between the new Remote Desktop client (v11.2) and your gateway server.
A very common fix for this specific problem is to clear the app's cache. Could you try this on one of the iPhads?
Go to your device's Settings > General > iPhone Storage (or iPad Storage). Find the "Microsoft Remote Desktop" app in the list and tap on it. Then, select "Offload App" and confirm. After that, just tap "Reinstall App" to get it back. This clears the data without losing your saved PCs.
This often resolves these kind of update-related glitches. If that doesn't do the trick, the next step would be to check if your gateway server needs any specific TLS security settings updated.
Let me know if the offload trick works! If this answer helped, please mark it as "Accepted" 🙂