Eplus is special in being the only German operator to let their prepaid subscribers access their 4G (LTE) network. That's why Aldi subscribers can use fast 4G service, though eplus' 4G coverage is still very limited and even in launch cities (Berlin, Leipzig, Nuremberg. Munich, Cologne, Bonn, Frankfurt and Stuttgart) patchy.
Unfortunately the frequency eplus uses for 4G today (LTE frequency band 3, former GSM1800 band) is not supported by your
Samsung Galaxy Mega (SGH-I527), which is targeted at the American market and so mostly supports American 4G frequencies instead (LTE: Bands 1,2,4,5,7,17). So despite your phone is 4G capable it can't get 4G service in Germany unless eplus starts LTE service in LTE bands 1 or 7 in which they own spectrum but haven't deployed LTE to yet. As of 3G (UMTS) and 2G (GSM), the Galaxy Mega is fully compatible with all German networks.