I'm sorry, I've made a mistake with 'the'. In fact, some names of networks contain 'the' and others don't. It's a matter of names, not grammar. So the BBC, the ABC (in Australia) are networks, as are ITV, Fox, and ABC (in the USA).
With this adjustment, we can treat network names and station names the same: I heard/saw it on the BBC, on BBC 2, on Radio 3, on the ABC, on ABC.
For the building, such as Broadcasting House in London, yes you would use 'at' because it's a building, and 'the' because that's part of its name: meet at the BBC, meet at Broadcasting House, meet at Harrods, meet at the Empire State Building.