Thieving peoples money

I ordered a wifi router from them end of May 2021 then my first bill arrived 26 June which I paid and I sent them proof of payment then I asked to upgrade my acc to the R599.00 plan which they did then they send me my statement saying I should pay R1005.00 which I don't get as the other 2 installments of R199.00 has been paid and proof was sent to them. Then I contact them and they just don't answer the phone I have spoken to the lady who set up my acc and she also can't assist me. These people take money for no reason.