Neck Curls and Neck Extensions at the gym. I do them 2-3 times a week and already see progress despite doing it for like a month or two. Definitely worth it.
Rep range for Neck Curls is 12-15 and for Neck Extensions 15-20. Full ROM and controlled Negative. I went from 5kg plate to 10kg plate easily. I use a beanie at the gym when i do them so the plates sit more comfortably on my head, on my rest days where i dont lift weights but just do cardio.
The Neck Extension movement will always be stronger than the Neck Curl, so expect to have smoother progression for it. After you get strong on Extension, consider buying a Neck Harness online, but make sure it's quality.
I find Squats, Deadlifts, Barbell Rows and Shrugs all help with neck training indirectly (especially when looking forward while performing the exercises) and it's how many lifters got a big neck without isolating it.
For more Neck Training information you can check out Alex Leonidas and Hersovyac on Youtube.