Suddenly slow home internet.

I got this gateway on January 5th and it was slow at first, but then jumped up to 120Mbps, so I thought I may be able to finally cancel my $60/month 200Mbps cable internet, and called them to do so. They offered me another year of $45/month 200M, but I persisted and then they offered me a year of $35/month 100Mbps speeds, but I already had this device and felt committed so I am returning their modem tomorrow and my service with them ends on the 20th.

NOW, however, my internet just went out completely last night, and repeated unpluggings of the gateway did not work, and the final one did, but speed was 1Mbps. I called and talked to someone and they diagnosed it from afar, and then had me turn it back on and test the speed, and it was now 106Mbps. They claimed I was on a problematic tower that they knew about, and he was now "locking" me into the better one, and this shouldn't happen again.

I got on this morning, and speed was back down to 1Mbps and even 0.26Mbps!! I don’t know what to do, my wife and I both work at home, and need consistent, reliable internet. 

I don't now know if I should call back my cable internet company and tell them I will take them up on their offer after all, and send this T-Mobile device back, or stick with it and try to glean some tips from this forum, but I don't know if it's worth it. How difficult is the return process, and what happens with my bill if I'm supposed to autopay the first one on the 21st?