How to boost 3G on my Android smart phone?

[Jasmine]

I run an art market and we use a Paypal Here card reader, which links to a mobile phone app and needs the internet to function.

Where the venue is, the 3G is really intermittent and unreliable, so sometimes we can take card payments and sometimes we can't, which is embarrassing to then have to turn people away and tell our stall holders the machine isn't working so they can't sell that piece of work they just got someone to buy.

Is there any way of boosting a mobile's 3g signal? Like a dongle for smart phones sort of thing? I have tried googling the problem but nothing comes up that I can see...

ANY help greatly appreciated!

J x