DNS

Model FAST8688W wifi router.

Are we allowed to change the routers dns? If so how? I would like to try others for my gaming. 

And how would I connect to the top C Port LAN. I called and spoke to a tech from t-mobile that said we could connect for multiple uses even direct internet, (like using an ethernet cord) except a double end C cord.

I would like to know what can be done connecting my S21 Ultra Android 14 and detailed instructions if possible or a link somewhere. Anything would be useful. 

Thank you for your time and efforts. 

Good day.