Help! Before I open the box, is it true this T home internet will replace my Spectrum service or should I return it.? I'm hearing negative reviews!

Like with all devices and internet services, there are good reviews and bad ones, and opinions change, too.  I don't know about Spectrum, but I replaced Cox with T-Mobile and currently have no regrets after less than a month of usage.  My internet speeds went from 181 download to speeds like 250-350. The upload speed went from always 5 to 22. 

I have had no outages, like Cox had all summer, and have streamed Prime Video, HBO Max, Hulu and YouTube with no problems on my Sony TV.  I did have to go to my Wi-Fi settings on the Sony at first, however, and sign in twice with my network password.  (I don't watch "live" TV, so can't comment on that. Plus, I don't game.)  My Panasonic Home-Hawk devices all work as usual, as does all my Echo dots. 

My eero 6 + mesh system had no problem going from the Cox modem to the T-Mobile gateway, either. The only negative things so far is it looks like the first T-Mobile internet bill is going to be a double one, which I never saw mentioned anywhere until I got the email about the matter; and I have had location detection problems with one website.  I was able to resolve that with the Edge browser by changing various location settings, but have not been able to resolve it with the Chrome browser. With Firefox, I just had to click in "allow" in a pop-up location box.

Since you have the T-Mobile gateway, I would just go ahead and try it out.  If it doesn't work for you, you can send it back to T-Mobile and go back to Spectrum.  I will go back to Cox, if T-Mobile starts to fail me and I can't quickly resolve the matter, because Cox was on the street doing repair work this week, which made the landline/internet modem cease continuously failing.  The price of Cox and T-Mobile are about the same, too. I do much prefer a totally wireless internet connection, like T-Mobile, however.