I just started having this problem.
I did this exact procedure (removed the app, restarted Roku, reinstalled app)
I get the error "Network Error! Please check your internet connection"
My subscription is directly through Sling.
Other Roku apps work fine.
Sling works on my phone.
Roku > Settings > Network > Check Connection says everything was successful.
I checked for Roku software updates and am up to date.
Roku Model: 3920X - Roku Premiere
software version 12.5.5 build 4174-91
Please help