Internet really slow

I have 2 prepaid Neotel internet modems. My internet is really slow and sometimes unusable and frustrating. I called the call centre to enquire and was informed that I have full coverage in my area and that a technical consultant have 48 working hours to look into this - 6 days!!!!! ... 6 days later somebody calls me to tell me ... ohhh that devices is to old you have to buy a new one!!! Buy a new one??? And what guarantee do I have that the new one will work .... and how often must I replace them?????