Earlier quoted context omitted.
Check out https://fast.com from Netflix. It's a speed test based off of Netflix's servers, so it will tell you your bandwidth to Netflix at any given time. Then complain to your ISP if that doesn't match your advertised speed.
When the advertised speed is "up to XX Mbps" and you occasionally do get those speeds, then what is the proper complaint? They are providing "up to" the speed purchased. It works very well in the provider's favor.
FWIW, even if you don't complain, it will help Netflix gather data that they can use to complain on your behalf.