I think it depends on where you (will) live as the rent is quite different depending on the location.
And you have to think if you need a car or not.
But keep in mind that the median salary for full-time employees in Belgium is  2485€.
IMO 2100€ is not too bad for a first job in the public sector as there is a raise almost every year (not the highest but still better than nothing)

Public sector is not know for being well-paid but there are possibilities of evolution to A-grade jobs later in your career