Anyone can shed any light:

Why, in Motijheel, in the same room, 3m apart, a shared 512kpbs BLION dongle (prepaid) is faster than a BLION indoor Zyxel 206M1R modem on a 1mps shared package (postpaid)?

Same provider.
Same signal. Then what is the culprit?

Both the dongle and modem work superbly well in other parts of the city like Gulshan, Banani, Dhanmondi, Badda.

Should I try moving to other part of the room?