Let’s Encrypt Root certificate expired on old Android tablet. How to fix?

I have an old Android tablet, and websites with Let’s Encrypt SSL certificates no longer works.

On Let’s Encrypt’s web site they basically just says it should still work
But in my case, it doesn’t, I get “NET ERR_CERT_AUTHORITY_INVALID” error. Date on the device is correct.

Does anyone know if there’s a solution to this? Shouldn’t it be possible to install the updated certificate manually? If so, exactly where can I download it, and how is it installed?