Thanks for posting to the forums!
Since reinstalling the SlingPlayer application on your iPhone has not resolved your issue, I would recommend resetting your Slingbox 350 to factory settings and then setting it up again through Watch on slingbox.com.
This article will guide you through the factory reset process for your Slingbox 350:
Once your Slingbox is completely reset, you will need to go to Watch on slingbox.com to setup your Slingbox.
Once your Slingbox has been setup, I recommend removing the SlingPlayer application from your iPhone and then reinstalling it again for good measure.
Let us know if you are still unable to connect to your Slingbox after you complete the Slingbox reset process.
Sling Media Moderation Team