I played with the app heavily for few days.
I have a Tailscale network and syncthing syncs over Tailscale IP addresses. For devices that have static IP addresses, also encoded in syncthing GUI (like tcp://100.xxx.xxx.xxx:22000), all other connectivity options (global and local discovery, relay, and NAT transversal) can be disabled. This set up worked well in my network, except for iPhone. The desktops devices connect quickly, but the iPhone had problems connecting to any peer.
This app does not have functionality to specify a static IP address for the phone, or other peers. However, you can nevertheless provide the Tailscale IP address of the phone in other peers. Ideally, this should be enough for other peers to find and connect to the phone app almost instantaneously (permitted by Tailscale ACLs).
That said, if all connectivity options are disabled in the phone app (regardless of the connectivity options enabled in the other peers), the app takes a long time to connect to other peers, 30 second to few minutes. The amount of time is also random, and can be short or very long. This makes no sense, since the peers have the IP address of the phone and should be able to connect to it almost instantaneously.
I played with different settings in the app. Indeed, the connectivity options in the app can all be disabled, except for the global discovery. With that enabled (regardless of the connectivity options enabled in the other peers, which do not matter because other peers have the static IP address of the phone), the app connects to other peers in few seconds which is acceptable. However, global discovery seems unnecessary when syncing over VPN, and I prefer if it could be disabled.
I used a custom config in the root folder of the app (specifying the static IP addresses for the other peers, etc), but could not get it to work well.
Currently, there is a need to expose more of the useful Syncthing options in the web interface (such as support for specifying the IP address of the phone and other peers, required when syncing over a VPN), or support for custom configs, and test that it connects to peers fairly quickly in different cases.
Considering iOS restrictions, I am not quite sure. Resilio Sync iOS app connects quickly and syncs fast. Unfortunately, itâs closed source, so itâs unclear how they do it. Since itâs closed source, I canât use it, because its security can not be verified.