I checked online and it was a huge sum for unpaid years at class 3.
I'd been paying class 2 over the years I've been working here, so I wrote to them, not mentioning what it says online, and as i expected with working in Thailand, they wrote back quoting a very reasonable amount at class 2.
I would therefore recommend the OP writing (or calling) to get an assessment, as the website assessment seems to assess on the basis of working in the UK, and act quickly as there is a 5 April 2025 cut off date for backpaying some of the earlier years.