Is there an app/website that tells us possible flight/rail routes to go from source to destination? for example: if I search from Mumbai to Kanpur then it should give suggestions like Mumbai to Delhi via flight and then train from Delhi to Kanpur

Another example, confirmticket app - when you are looking for reservation and it isn't available then it suggests alternates.

The Beckn protocol aims to solve/do this. I'm not sure if it has a released app. I remember some app using the protocol being live in Kerala few years ago.

How about Rome2rio https://www.rome2rio.com/map/Mumbai/Kanpur