If you do not have a local WiFi network to use for Codebook's Desktop WiFi sync service, and you are using Codebook on both iOS and macOS1, you can set up what's called an Ad Hoc or computer-to-computer WiFi network to establish a temporary network for Codebook Sync.
The steps for this vary a little bit depending on your macOS version. Apple has very good instructions available for all recent versions of macOS (10.13 to 12).
Now that you are connected to your Mac's Ad Hoc Wi-Fi network from your iOS device, you should be able to use the Desktop WiFi sync service. Launch Codebook on your iPhone or iPad, ensure Codebook for macOS is running and configured for Desktop WiFi, and start a sync from your iOS device (tap the Start button at the top of the Sync view).
1. You may be able to use this technique with Android and Windows devices, but it's quite complicated and we can't support or recommend it at this time.