You have to check T-Mobile Poland. This is T-Mobile USA.
I take it that you bought the Galaxy Grand Prime in the US and are now trying to use it in Poland? The Grand Prime is a poor phone to use outside North America. The only international bands it has are GSM 900 & 1800 (2G). All the 3G/4G bands are North American. It's sort of the reverse of all the Americans who buy phones intended for the Chinese or Indian markets and wonder why they don't work well in the US.