Start by separating the network from the provider
Play the same channel on a phone using cellular data. If it is smooth there and stutters on the television, the problem is inside your house and no amount of complaining to support will fix it.
This one test saves most of the time people spend guessing, so do it first.
Fix the Wi-Fi before anything else
The 2.4 GHz band is shared with microwaves, baby monitors and every neighboring router, and live TV is unusually sensitive to the resulting jitter. Move the television to 5 GHz, or better, run an Ethernet cable to it.
A $10 cable outperforms a $200 router upgrade for a fixed device like a television. If a cable is genuinely impossible, a pair of powerline adapters is the next best option.
Check whether your streaming stick is the bottleneck
First-generation Fire TV Sticks and budget Android boxes have very little RAM. The app has nowhere to buffer, so any network dip becomes a visible stall.
Cap playback at 1080p and close background apps. If stalls persist on a five-year-old stick, replacing the hardware is a more effective fix than any setting.
Compare peak and off-peak performance
Run the same channel at 9pm and again at 11am. A large difference points at ISP congestion in your neighborhood rather than the provider.
Note what you find before contacting support — a report that says "stalls only between 8 and 11pm on 5 GHz Wi-Fi" gets a useful answer, while "it buffers" does not.
When it really is the provider
If a specific channel stalls for you off-peak on a wired connection while everything else is fine, that channel's source is the problem and support can reroute it.
This is where a published uptime SLA earns its keep: it converts an argument about whose fault it is into a service credit.