Usually yes, but it depends: https://pierrekim.github.io/blog/2016-11-01-gpon-ftth-networ...

Also the authentication might rely on weak secrets. I know my ISP provided FTTH router has a six letter password and a guessable username (derived from my last name), and I can't change either.

Though the research is quite old now. Couldn't find anything recent specifically for DT.