A cold email campaign - no? Too much work, too little return.

The timeframe is too long.

The longer the period between when people sign up and when the webinar is held - the fewer people will attend.

At the point of sign up, people are excited. Over time, that excitement wanes and you're left with folks who're busy and may have (what they consider) better things to do with their time so your webinar and the reminders get pushed to the backburner.

The single most impactful thing you can do is shorten the time between registration and the actually webinar. Keep it at one week and see how it goes. A tool like WebinarJam (see more here: https://www.growthboost.co/blog/webinarjam-review/) gives a lot of flexibility.

If you don't mind an automated webinar then a tool like EverWebinar will allow you to do just in time webinars. That is, within 15 minutes of signing up, the presentation starts. That can boost your attendance rate to over 50%.