Telegram is a nice messenger app but it is neither as secure nor as trusted as Signal, which I use for any communication that requires actual privacy, for a very simple reason. See the below quote by an actual expert:
They use the MTproto protocol which is effectively homegrown and Ive seen no proper proofs of its security, Alan Woodward, professor at the University of Surrey. Woodward criticized Telegram for their lack of transparency regarding their home cooked encryption protocol. At present we dont know enough to know if its secure or insecure. Thats the trouble with security by obscurity. Its usual for cryptographers to reveal the algorithms completely, but here we are in the dark. Unless you have considerable experience, you shouldnt write your own crypto. No one really understands why they did that.