Netlify returning 404 when connecting to Atlas MongoDB

I'm trying to make a Netlify app that posts data to an Atlas MongoDB, and while I can post to the DB when I run my page from localhost, Netlify is returning a 404 whenever I attempt to post data to the DB. I know it is not an issue with Atlas's whitelisted IP addresses because I have whitelisted all IP addresses for the time being. I suspect that this has something to do with Netlify not properly reading or running the env.process that I'm using to store my Atlas information, although I am not completely certain that is the cause. When I run it locally, I have my config set up to simply use the Atlas information directly rather than relying on a .env file. I'm using mongoose to connect to the DB, and the connection portion of my code is the following in my production build:

mongoose.connect(process.env.MONGODB_URI || "mongodb://localhost/dbname");

This has not been working, but on the working copy that I run from localhost, I use:

const uri = `mongodb://atlasDB:@atlasDB-shard-00-00-ot2tv.mongodb.net:27017,atlasDB-shard-00-01-ot2tv.mongodb.net:27017,atlasDB-shard-00-02-ot2tv.mongodb.net:27017/test?ssl=true&replicaSet=atlasDB-shard-0&authSource=admin&retryWrites=true`;
mongoose.connect(uri);

I have configured Netlify to have a MONGODB_URI build environmental variable of mongodb://atlasDB:@atlasDB-shard-00-00-ot2tv.mongodb.net:27017,atlasDB-shard-00-01-ot2tv.mongodb.net:27017,atlasDB-shard-00-02-ot2tv.mongodb.net:27017/test?ssl=true&replicaSet=atlasDB-shard-0&authSource=admin&retryWrites=true

I have replaced PASSWORDHERE with the actual password in both instances, but the Netlify build environmental variable does not feature string quotations around the value when viewed in the entry field on the Netlify website. I tried putting them in, but it seemed to make no difference, but I may have simply not waiting long enough for the change to take effect.

Aside from Mongoose, I am not running any other dependencies that should have any effect on this problem. The project deadline is in a couple days, so any help with this would be greatly appreciated.