You may hold down the right mouse button and circle the area to select the units. Sometimes a "triangle" motion works better than a circle motion. This will make a gray shadow and select all the units in the loop. In the case of planes: Having the circle include the airbase or carrier they are stationed from is necessary. You will then need to use the split command to separate the planes from the carrier or units at the airfield.

Or, For units with a known designation, such as AB12, you can use Ctrl +F and type in the unitID to select the unit. You may be able to determine the designation from newspaper reports or from the unit list as if you were going to trade an army to someone in diplomacy menu. All of your units will start with the same letter code.

Or, The units can be selected by cycling through the units with the left and right arrow keys until the desired unit is found. Typically if you start by selecting a nearby unit and use an arrow key you will get other units within view. In some cases reloading the game will freshen the list and sort order, and enable you to find the unit easier with this method.

This area is known as the "map seam" because of the way that the map joins together to allow continuous play around the world. This does lead to difficulty in clicking directly on the units while they are in the overlap area. Mr Prince outlined the alternate selection steps above.

I will add for mobile users that scrolling so that the unit is on the very edge of your screen will allow you to select them easier.