In a novel I am writing one of the antagonists travels to Earth because at the end of the novel, he travels back in time and brings himself to Earth. I'm sorry that it's a little confusing. Essentially he is transported directly to Earth from his own planet by a wormhole opened by himself who has traveled into the past after being brought by himself (the same event). Essentially, he is brought into that dimension and needs to reconcile the timeline, I think, but would this work? Or am I missing a law of time-travel?
There are no laws of Time Travel
The issues is paradoxes and how you handle them. You cannot have paradoxes.
1: Time is fixed. You cannot change time. If someone goes back, it was they always were going to go back. They will always do exactly what they were going to do. In the movie 12 Monkeys, the time travelers are going back to get information to cure the virus in the future. They already know they cannot stop the virus from starting.
2: Time sprouts off a new branch. When you go back in time, it makes a new timeline. You can go back and kill your grandfather because you're already been born in another timeline.
3: You don't actually travel in time. You actually hop infinite dimensions to an identical dimension but less chronologically advanced. Again, you don't need to preserve the timeline because you were born elsewhere.
This is not necessarily a problem.
What you've described is a tautology: if X, then X. What you need to avoid is a paradox: if X, then not X.
The problem with tautologies is getting X to happen at all. That becomes part of your premise, in this case, unless there's a "first timeline" where the portal was opened in a different way.
If you do take this path, be aware that it's an unstable equilibirum: a small change can push it from "everything works" into progressively larger problems. Consider if the future part of the time-portal is delayed by a minute, which elts your character get into the habit of being late, so they're now two minutes late to open the portal ... and eventually are running late to something are are hit by a truck.
This is why most stable time loops use the "one and only way" fixed-future model. The author is already sculpting one exact past and future, and doesn't complicate it further.
(The problem with paradoxes is that they stop themselves directly.)