This is happening because the 1.25" nosepiece of the T-ring you bought is smaller than the camera sensor.
A full frame camera sensor is 36 x 24mm.
A 1.25" eyepiece exterior dimension is just 31.75mm, and the widest interior dimension is usually 29mm.
This is why you can see the dark edges on the width of the sensor (36mm), but not the top and bottom (24mm). And of course the diagonal dimension is larger than even 36mm.
The solution is to get a 2" nosepiece for your T-ring: https://www.amazon.com/Serounder-Astronomical-Telescope-Eyepiece-Photography/dp/B07QV8RNB8/
However, if you only have a 1.25" focuser, then I'm afraid you're stuck with vignetting in that telescope / camera combo. Options would be to downgrade the camera to one with a smaller sensor whose diagonal size is smaller than 29mm, or upgrade the telescope to one that can actually support a 2" focuser.
Note that adding a 2" focuser to a telescope designed for a 1.25" focuser will not fix the problem.