Hi Sean,
Thanks for reaching out! I’d be happy to help you with this.
I think the issue here is that the IDs for venues are associated with events, but you will first need to query the venues to get only those located in Frankfurt, and then pass those venue IDs as an array to meta_query.
Try replacing the code you have here with this snippet:
https://gist.github.com/jentheo/1be5502370030a3d52061e3e751a5f4c
Note: I assume you have defined $paged somewhere outside of the query, but if not, make sure to do that so you don’t get an error.
Let me know how that works for you!