Spoke to the guy at Otto, and going to try this model: http://www.otto.co.za/products/antennas/gsm-3g-4g-lte/omnidirectional-mimo-antenna-7db-gain/
If it doesn't work too well, I'll consider getting the Poynting one and returning this one. Good thing about this model is it doesn't have to be mounted externally, and guy said it should have at least 3 times the gain my internal B618 huawei antenna can manage
I'm not keen on omnidirectional antennas. You get much better gain and less noise with a dipole or Yagi