As already said Flanders is the Dutch speaking part of Belgium. Drawing an east-west line just passing the southside of Brussels it divides the country in Flanders, the northern Dutch (Flemish) speaking part and Wallonia, the southern French (Walloon) speaking part. But it is also a political division each having their own government and also own public transport facilities for buses, trams and metro’s. So for Flanders is this only De Lijn and services public transport in and between Antwerpen, Gent, Brugge, Mechelen and Leuven for instance. Meaning tickets, so a 10 ticket pass too!, bought in Brugge can be used in all other mentioned cities too and vice versa.
Bilingual Brussel(s) is an exception having it’s own public transport MIVB/STIB where you can not use the tickets of De Lijn. As the Grand Place is at walking distance there from Brussel-Centraal railway station you likely won’t need tickets there.
But trains are national with their own tickets, you can’t use De Lijn tickets for train travel. For the rest De Lijn has nothing to do with public transport in the Netherlands. Trains from Utrecht to Belgium are international, so buy your tickets in Utrecht or online in the Netherlands.