You can run your Lambda function within the same network as your other servers. It just appears as a new IP address inside your network and can call whatever you permit it to.

our MongoDB is running on mlabs.

So but you mention you're connecting without lambda - so where are you connecting from?

he probably meant 'within' instead of 'without'. myself to use aws-serverless-express and connect my lambda hosted in us-east-virginia to a mlab (now Mongo Atlas) mongo database hosted in the same us-east-virginia amazon region