Serverless Deploy command causing 'cannot resolve' DB connection string error - node.js

Question

I have a serverless API that's no longer connecting to the DB. Initially when it was built, the DB was a self-hosted instance in AWS. I recently moved the DB to Atlas and now the API can no longer connect to the DB.
When sending queries via postman, I'm getting invalid credentials error. I'm assuming the error is due to some outdated packages. So I updated them, pushed the code to the repo, then the pipeline zipped it up and stored into S3. Now when I run sls deploy I get the following error:
Cannot resolve serverless.yml: Variables resolution errored with:
- Cannot resolve variable at "provider.environment.MONGODB_URI": Source "file" returned not supported result: "undefined"
The connection to the DB is a Lambda Function, so I'm not sure why its looking locally for the connection string and not for the lambda function.
My serverless.yml file looks like this:
service: aws-lambda-api-service
variablesResolutionMode: 20210326
frameworkVersion: '2'
provider:
name: aws
runtime: nodejs12.x
lambdaHashingVersion: 20201221
stage: dev
region: us-west-1
environment:
MONGODB_URI: ${file(./config.js):fetchMongoUrl}
and my config.js file looks like this:
module.exports.fetchMongoUrl = () => {
// create / fetch dynamic data here (e.g. call an API)
return process.env.MONGODB_URI;
I understand that this is sort of a loop that doesn't contain the connection string. My question is, what do I need to do for it to look for the lambda function that handles the connection?
I just want to deploy the updated packages in the zip file so that I can see if it fixes other connection errors I'm receiving when attempting to send queries via Postman.
Any help will be greatly appreciated.