The getInstance() method seems to indicate you are trying to use a singleton
pattern.
Generally with that pattern, the getInstance() method should be static, but none of the other methods on the class.
You obtain the object via the call to getInstance() and then invoke the non static methods on that object.
so
The database code looks a bit dodgy as well.
What is the ConnectionsManager class?
You get both a connection and a statement from it, but then only use the statement.
The general way to use it would be to obtain a connection, and then create the statement from the connection object.
I would also recommend using prepared statements over sql strings like you have to guard against sql injection.