How to configure the display manually on ubuntu core?

I a tv or monitor don’t show regularly, for exampel showing little or huge black borders, then I go here

: sudo nano /boot/config.txt
how to manual configure the tv and monitors on ubuntu core if it automatically don’t fine the best configuration?

I remember this for example:
CEA should be used for TVs
DMT should be used for computer monitors.

or also I am using

sudo raspi-comfig