Theres a little confusion.
The difference between subjective and objective is the difference between who is the best rock band, and what is 73 x 11.
Subjective things are based on personal opinions and feelings and may very well differ from person to person. Objective things are based on exteternal facts and evidence, and generally don’t very much need opinion to be given.
If Something is based on facts, those facts are broadly agreed, and the logic is determined to be valid: then its objective.
Those facts could end up being disproven, or the logic turn out to flawed in some way: but until that is determined - the principle is still objectivez
In the same way, 1+1 = 2 is objectively true, and not based on feelings.
While it’s possible that we’re all mad, or that you’re the only brain in the universe and maths is all wrong: it’s also possible - and likely more probable that it isn’t. Until you show that all the facts and logic of maths are actually based on feelings or some weird collective delusion: it remains therefore objective.