Maybe. Some of the newer asus and tplink routers that support 3g dongles can use these in a backup configuration.
The problem is the "automatic". What is considered down and how do they detect it is the hard part. On commercial connection you can run a routing protocol over the connections to detect a failure. In these consumer routers I think it pretty much can only fail automatically if the ethernet port goes down. This would require the mtnl router to detect the dsl go down and take the ethernet down to tell the other router. How would you detect as you state "low bandwidth"
I suspect you will have to manually switch it over.
The DD-WRT firmware also supports 3g devices on these same routers. I have seen discussion of writing custom scripts to do things like ping and then issue the command to cause it to switch if it detected a outage. Writing your own scripts for dd-wrt is getting a little advanced for most people but it appears you can accomplish what you want if you work hard enough at it.