Account downgraded to basic despite payment

Hi I’m looking for advice as I have contacted Zoom numerous times and just gets the same response which does not address the issue. 

I paid to renew my Pro subscription at the end of January, before my contract expired. Despite this Zoom downgraded my account. The payment shows on my account as a credit less £9 or so. Each time I contact Zoom they just say renew my account. They ignore the £146 credit on my account. I have tried to renew my account but when I do so it does not credit me the £146 so I do not want to renew until the £146 is credited or refunded, but Zoom are ignoring correspondence sent via their help line. 

Advice as how to resolve this is much appreciated.