Internet cuts when it rains

When it rains the internet connection drops every single time. Their website is useless because when I try to log a fault online, I fill in the details and click submit but nothing gets submitted. So I must just be stuck with no internet whenever it rains. It makes no sense because fibre is not supposed to be affected by weather yet they have managed to achieve this great feat of even messing that up. And the problem is not with my ISP.