Devices will roam across networks if equipped with a suitable roaming SIM card. For best results, try to find a SIM that will support as many networks as possible. This increases your chances of successfully uploading, and reduces battery usage.
When the device finds that it is unable to connect to the last known network, if has to scan for all networks in range, and attempt to roam onto any available network that has not refused it recently. This scanning process, and any connection attempts produced, use about twice as much power as a GPS fix attempt, and can last for minutes. This means that when out of network coverage, up to 5 times more energy can be spent on failed upload attempts as is usually spent on a successful upload.
By default, for example, the Oyster spends up to 3 minutes per connection attempt. When in coverage, this gives the device a good chance of successfully uploading in weak and changing signal conditions. However, if your device is frequently out of coverage, you can improve its battery life by further limiting the upload timeout. This is possible in firmware versions >= 1.3, using the Upload Timeout parameters.