I am also experiencing similar high ping starting around Christmas 2023.

My ping was consistently between 170-220ms back in December, but it has since increased to over 500ms. The lag is used to only occur around 4pm-1am SGT but it has gotten worse lately.

I have contacted my ISP to investigate the issue, but they have been unable to resolve it. This problem appears to be specific to Final Fantasy XIV, as I have not encountered it in any other games.