Sim help

Can anyone help please? I have a problem with thieves on my allotmentment. So I decided to set up a cctv. There is no electricity so I bought a solar powered camera. I am 77 years old so not brilliant with technology  so I got my son to set it up. I bought a mobile MIFI router and fitted a smarty data only 15GB sim for £9 per month. Brilliant it worked, but I have,ny even got it to my allotment because it's been to hot to work? Its working brilliant in my back garden but after only 2days and night it's used 14.75 data 😡 I am ony on basic state pension so I need an affordable plan but I was pushing it at £9 per month , Help.🤠