Thought I'd throw this in the mix... a suggestion that you may already know.
Traffic and engagement come from one thing: Content. Good content.
Over time, forum content becomes self-perpetuating from the active members. But, in the beginning, as the forum owner, you may need to prime the pump. So, I think it helps a lot if you can create content.
Whatever your forum topic is, hopefully you're knowledgeable and can create thought-provoking content, which will lead to discussions / comments. And, it will inspire others to post good content.
It takes a lot of TIME and ENERGY. But, I think it will pay dividends... eventually. That's my story and I'm sticking to it!