Well, I confess that I was surprised that you were using WEP, these days this is very rare, and as @BlakeRichardson said, this is insecure and could perhaps be the cause of the problem, but the truth is that without the details of the other configurations this is nothing more than a guess
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
Please, if this post was useful, leave your kudos and mark it as solved.