Internet Speeds Cut by Half

My speed was 1 gig up until my recent payment where they charged me $187 for the extra router I never ordered, which I returned but the local store I took it to never shipped it back to the warehouse it was supposed to be sent to. Now its 75% slower. This is business fraud and financial fraud.
The FTC needs to reel in T Mobile and fine them a few million $ to remind them that fraud is unacceptable business in America

Check the cellular metrics and see which bands the gateway is on. It may have changed the connections to different cells. If you check the advanced information for the cellular metrics that could explain the changes. It may have had a lock on an n41 cell source and now is on n71. That might account for the change BUT maybe not. If there are more users on the cell and there is some congestion then the speeds will be reduced as the bandwidth throttling to more evenly distribute the available bandwidth would take place.

Weather can have an impact upon the cellular communication as radio frequencies are not designed to travel through water but air. There are multiple possible factors but given the drop as you have described I would guess the signal source lock has changed. No data to know for sure and no cellular metrics before and after the change so no way to be sure.

Yes, inexplicable degradation of speed down to 20 MBPS - guessing it switches to LTE and doesn't do as well. Some days over 100 MBPS. Not very reassuring. 

mine dropped to 40down 5up from 300/120s. and yep, gateway no longer connects to 5G. Now only grabs B71.  Used to connecct to B66 and N41. 

Oh yes thanks. I see now. My gateway is at b12. I expect there is no way to insist on 5G.