Att is a scam! Slow internet, terrible data!

I switched from Xfinity and Sprint a couple of months ago, because I was told by the sales people I was going to get awesome service!! Ehh no, not at all! I switched and quickly the internet speed started to get worst, to the point where I know flip back and forth between data and wifi to try and get pages to load, because having both on seems to make it worst at different times. I have a new  iphone with lots of space so the phone is not the issue. What a joke! I had my xfinity modem under my couch and would get service through my tiny apartment, I have the att modem in an open centralized space for which I had to use my own cable because the tech that came to do the installation had only 3ft cables!!! Either way I get no service, I can stand right next to the modem and nothing, I have the app they recommend and this thing lies saying I have great connection yet I have to wait forever for pages to load?? With Xfinity there was no wait time at all! I reached out to att and was put on a different channel, over and over again. The service would improve for a minute and then back to nothing! The worst part is if Xfinity ever failed in my bedroom which is the place I get worst signal, I had Sprint and it would cover without any loading issues. But now I have this horrible service from Att on both wireless and internet! Can I use a router or something to help my case? I don’t want to give Att anymore of my money getting wifi extenders... has someone had this issue resolved? I see lots of people with the same problem...