When trying to watch live TV, I get a provider service error code 012-999-403.
My provide is xfinity. I’ve reset the Roku device, but the error still occurs.
Reboot router as well. If the error persists it's probably just Comcast rebooting servers or some such. We had a lot of network maintenance in our area recently and at least three different periods when similar errors would pop up a lot. Are they working in your area? Any notices come in the mail?
Sometimes you just have to wait it out. It can take days or weeks (not in total outage, but temporary outages) for Comcast to sort out the issues.