T-Mobile USA review: Not the speeds I signed up for

Purchased home Internet modem. Happy with the 2 guys, (Jake and Brendan”, who helped me set up a 100 mbps plan. Was told it’s just basically plug and play but I need help determining where I plug my phone, tv, and security cameras. Plus, my speeds are between 5 and 15mbps. Called for assistance and a useless employee answered and told me I could drive to their store for help or I could call 611, which is the help line for T-Mobile cell phone users but I have AT&T! There is another useless employee at that company. It’s hard to avoid them all cause sometimes they’re the only ones there. Looks like I have to cancel my account cause it’s not looking any better than my old ISP. Would have rated 1 star if not for Jake and Brendan.
When I called customer service, I spoke with Rose who made it sound like 15mbps was acceptable and speeds of UP TO 100mbps doesn’t mean 100mbps. I signed up for the plan that offers DL speeds of 87-318mbps. She used the analysis of when you’re in traffic…
So unhappy with T-Mobile already.