Firstly, I must say I am impressed with your 'thinking out of the box' approach. But problem is, like some have said, it is a bit too complicated and may be against the law (for vandalism or 'advertising without a permit'). HOWEVER, I am not going to lecture you about that since I think you know what you're doing.
The reason for the low conversion rates is likely due to trust factors. I mean, if I see a random advertisement in the streets that shows a website link and then it asks me for my info (email, that is) I would have the impression that it is a shady/scam website no matter how convincing the opt-in page says and looks. Unless it is a legitimate advertisement from some brand names I know (if you want to go to the blackhat way of using company's brand name and logos, that's up to you but it is entirely illegal FYI). Also, like others said... Perhaps it is inconvenient and hard for drivers to see your URL and to type it into their phones. So why not put a QR code in your advertisement too? This way, all they need is to scan it and it takes them to your site. You can find many free QR code generators online.
Also, not sure if anyone has shared this... when it comes to digital products, the likely buyers are people who are usually above 35-40 years old as they tend to be 'not so computer literate' and have high disposable income, therefore they are willing to fork out cash to buy it. Whereas for the younger millennial they have the mentality of "digital product? Yuck... Let me see if there's someone sharing it online or I can download from torrent or something". And if you put a $37 price tag for a digital product they are very unlikely to buy it, and even so, they are highly likely to be asking for a refund later. Unless, of course, the product has answers that they are really looking for - "Crypto code that helps you make millions trading crypto!", "This tea recipe helps you to lose weight effortlessly!" Even numerous popular CB products are providing physical copies now because these vendors know the game is changing. Also, it allows vendors to collect customer's info (won't go deep into this one).
Interesting fact - why do you think CB video and text salespage are so long and they spend most of their time telling you 'their' story? Well, to build trust and bond with the viewers. Then, the usual sales tactic - proposing some 'secret' that 'you can't find elsewhere' that you will learn when you buy their product.
Good luck!
P.S: Mind is still pretty hazy. Haven't had my coffee yet. So there may be irrelevant info in my post.
We won't tell you if CB works unless you elaborate more on your process 
