Custom DNS server

I have T-mobile's home internet with their 5G Gateway. I have my own router connected to it as I have multiple devices physically wired to my router. But its an old router and I want to retire my old old router and use the TMO device as my router. But I need to set custom DNS server as I have my raspberry pi as the dns server. 

Any way I can set up custom DNS server with the gateway?  I have checked the settings in the T-Life and HintControl apps. No ability to set custom DNS server available.

Any thoughts from the community?