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, in the Memory-Map app, tap the menu button, Settings, General Settings, and un-check the option "Automatic Map Download". With this mode set, the app does not use the internet at all, unless you request a bulk download, sync your overlay data, etc. Of course, you need to download the required parts of the map in advance, so there is no need for the app to go online. See Here.
However, when on a poor data signal, the phone may consume battery power searching for a signal, so enabling Airplane mode is also recommended. Or you can just disable the mobile data connection if you still want to be able to receive calls. If you scroll the map to an area that is not downloaded, while the data connection is turned off, you may 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.