For anyone else who’s come across the issue, unfortunately we found the only real option was to set up a server somewhere else to run the App Store Server Library. Then, you pass on the decoded and verified information from this server to your Atlas App Services app. We personally did this by setting up an AWS Lambda function to run the library.
This admittedly sucks, but for anyone who’s using Atlas App Services for a serious app, it seems inevitable that you might need to set up another server to run SDKs that aren’t compatible with Atlas functions.