Komga, Kavita, or any OPDS server. Your whole collection, readable in Longbox without importing a single file.
This is where nearly every failed setup starts. If you set your server up on your computer, its browser address is probably localhost:25600 or localhost:5000 — and "localhost" means this machine. Typed into your phone, it means the phone, which is not running your server.
What you need is the computer's address on your network, which looks like 192.168.1.50. On Windows run ipconfig and read the IPv4 Address; on a Mac look in System Settings → Network; on Linux run hostname -I. Most routers will also let you reserve that address so it never changes.
Longbox warns you if you paste an address pointing at the phone itself, rather than failing silently.
In the app: Settings → Comic Server → Komga.
Tap Test Connection, and save when it comes back happy.
Kavita works differently from Komga, and this trips people up: it does not use your password. Kavita identifies you with an API key that lives inside the OPDS address itself.
In Kavita, open your user settings and find the OPDS URL. Copy the whole thing — it looks like http://192.168.1.50:5000/api/opds/1a2b3c4d-... — and paste all of it into Longbox's address box. Longbox splits the key out into the catalog path for you, and you can leave username and password empty.
If you copied that URL from a browser on the server machine it will say localhost:5000. Replace "localhost" with the computer's network address before pasting, and keep the rest of the URL exactly as it is.
Settings → Comic Server → Other OPDS server lets you set the address and the catalog path yourself, with an optional username and password. If your server's catalog lives somewhere other than the default, put that path in the Catalog path box — it is the part of the URL after the address.
On your own network you can leave the protocol off entirely. Longbox assumes http://, which is normal and fine for a server sitting on your own Wi-Fi.
Reaching your server over the internet is different: iOS only allows plain http on a local network, so a remote server needs a full https:// address with a valid certificate. If you have put your server behind a reverse proxy with a real domain and certificate, use that whole address.
The first time Longbox looks for your server, iOS asks whether it can find devices on your local network. You have to allow it — a server on your own Wi-Fi cannot be reached otherwise. If you tapped no, turn it back on in the iPhone's Settings → Longbox → Local Network.
Longbox tells you which of these went wrong. Find your message here.
Nothing answered at all. Nearly always the phone being on a different network — check it is on Wi-Fi rather than cellular, and on the same Wi-Fi as the server rather than a guest or extender network. A VPN on the phone will do this too. Then check the server is actually running.
Same family, usually a wrong address or port, or a firewall on the server machine blocking the port. Try opening the same address in Safari on the phone — if the browser cannot reach it either, the problem is the network or the address, not Longbox.
The address was right and the server answered. For Komga, check the username is the email you sign in with. For Kavita, make sure you pasted the whole OPDS URL including the key, and left the password empty — a password in that box is not what Kavita wants.
The machine is there but the path is wrong. Komga should be /opds/v1.2/catalog. Kavita's path must contain your API key.
Something answered, but it was a web page rather than a catalog — usually the server's normal web interface. Check the catalog path.
A banner appears under your library title explaining which of the three it is: the server cannot be reached, it rejected your sign-in, or it returned an error. Tap the banner to go straight to the server's settings. Your library still lists everything it knows about, and anything you downloaded stays readable.