China Southern airlines make it impossible to rebook

Their website is so outdated. Their customer service is slow and it's impossible to rebook flights.

Over the course of a few days I tried to rebook a return flight. As I had cancellation insurance I was informed by one of their agents that it would be better to cancel the flight first and get 90% refund and then rebook.

Then they changed their minds and told me that I couldn't cancel the return flight as I had already taken the outbound flight. I let them know about my cancellation insurance for both the outbound and return flights but their response was the cancellation insurance for the outbound and return flight was booked at the same time so it was invalid.

Furthermore, if I wanted to rebook it would be 1400 GBP plus service fee for a flight that originally cost me 579 GBP.

They also said they tried to contact me by phone to help me resolve my urgent issue but definitely didn't.

I lost the chance to rebook my flight so I have to buy a new ticket.