@WB4IUY-
The only other player in this game is T-Mobile. Have you asked them to investigate whether they are blocking access to Sling's servers?
p.s. If T-Mobile is like most companies these days, don't be surprised if the front line support script readers don't understand what you are asking them. If you sense this is happening insist on getting passed to a higher level of support.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X