Don't buy traffic. Earn traffic organically.
Remember - PBN's are suppose to act like a real blog, so treat it like one.
Update your blog posts regularly. (minimum 1 x month)
Also build relevant backlinks to your blog.
You will start to see slight increases to your blog authority, and increases to your organic traffic.
Through this, your PBN will look like a legit blog - this is how you sustainably maintain a network.