I tried uninstalling, unlinking, restarting, reinstalling as mentioned previously and none of those worked. Here's how I got mine to work:
When you go to reactivate/relink your NBC account via the NBC activate website, make sure you have cleared your web browser cache on the device you are using- especially if you have used it before to activate.
I was not only having issues watching live tv with the NBC app (on multiple tv's), but with my HGTV, Food Network, etc. I always use either my phone or my ipad to activate my apps and link it with cox cable. I happened to use my spouse's phone to try the activation, which I've never used before and viola- it worked!
So to test my solution on my other tv's that were having the same issue, I cleared the web browser cache and cookies on my phone and ipad, reactivated/relinked using the respective activation websites and it worked. Now all my apps that were having live tv issues are working fine.
I only started having these live tv issues after the Roku TV update in December.
This worked for me, but hopefully this will help others resolve their issues too.