I'm hijacking the top comment to restate what I think may be the solution and why.
I think this is the solution (pic8.co)
It matches the Mirror/inversion theme. In fact it's a perfect match.
It reveals the 'up arrow', the only unknown character.
The 'up arrow' is the only character that matters, it points to the number 139. In other words the sign is just the address number '139'.
The 'up arrow' is a nod to social media where the puzzle will likely be posted/pondered and also the reward for the person that solves it.
That's my theory anyway. What say you?
Edit: Nevermind @RomanSalute has solved it... https://pic8.co/sh/jkDJdS.jpg
(post is archived)