Hi, there! Maggie, a Shopify Partner here :)
The best way to target a specific niche is to work on getting the backlinks and traffic from the specific places, that attract your target audience. So, you want to get backlinks from platforms that are visited by the buyers of phone accessories etc. The best way to deliver it is to invest in the best SEO package performed by experts in the ecommerce world. You can also invest the time to learn it yourself and work on your SEO daily.
Ecommerce Expert at Rock Paper Copy
Since I'm not clear as to how you want to target your audience, I'm going to take the freedom and write down the 3 best-known ways -
*Tip - For Facebook, do make sure that your audience reach is broad as the cost per click will be less. And then when you start getting results, you can always optimize your campaigns accordingly.
Lastly, when you do get these visitors on your website, you could then think about re-targeting them with Email and Browser Notifications.