Sounds very regular, may be worth contacting your ISP to see if that is their maintainence period or any other regular operation they may perform.
I am with a broadband provider in the UK called Zen, at 23:15 every Sunday our PPPoE session is terminated, and it takes a couple of minutes before it successfully resumes. Occasionally it will happen mid-week, however still at 23:15. At one point I suspected the provided FRTIZBox router, however the problem persisted once we moved to an MT992 modem and a Unifi USG.
I am with a broadband provider in the UK called Zen, at 23:15 every Sunday our PPPoE session is terminated, and it takes a couple of minutes before it successfully resumes. Occasionally it will happen mid-week, however still at 23:15. At one point I suspected the provided FRTIZBox router, however the problem persisted once we moved to an MT992 modem and a Unifi USG.