FileTunnel works completely offline, and you don't need any network environment. But you need to connect your device to your Mac with a USB cable.
Currently, FileTunnel doesn't support wireless connections.
No, the app will not send any network requests. Your data is completely processed on-device, and will only be transferred to your computer over the wire.
Use the highest specification cable compatible with your device, such as Thunderbolt 4 (USB‑C) Pro Cable.
Just file an issue on GitHub, it's easy.
If that happened, it must have something to do with the quality of your phone.