If you are in an area of marginal mobile signal, and you change maps while Memory-Map is trying to download map data, the app may freeze until the communication attempt times out. The best thing to do is just wait for the app to unfreeze, which may take 30 seconds or more. Then close Memory-Map and go to the settings app and turn off Cellular Data and Wifi. (On some iOS devices, using Airplane mode also turns off the GPS, so we recommend just disabling the data connectivity). If you scroll the map to an area that is not downloaded, while the data connection is turned off, you will get a message saying "Connection the map server has failed". This is fine: you will still be able to use the parts of the map that are present on the device.
Of course, if you download the required parts of the map in advance, there is no need for the app to go online, and you will avoid this issue. See Here.