Have you looked into Studio 3T? It's a MongoDB IDE with a SQL Import feature that supports the main SQL databases (Oracle, Microsoft SQL Server, MySQL, PostgreSQL): https://studio3t.com/knowledge-base/articles/mongodb-import-export/#import-sql-to-mongodb
Looks like you may be able to edit the PPPoE settings in the DB; I've not tested it, but I can see stuff like the username and password in there -
Download something to interact with the MongoDB on your PC if you dont already have something (I just downloaded a trial copy of Studio 3 T).
When you open it, you'll need to add in a new connection -
(I don't use Mongo, so forgive me using the wrong terminology below!)
Once connected, on the left hand side you'll see a database called "ace" expand that and there is a folder called "Collections". Again expand that and find the table called "networkconf". Double click and it will return the data from the table. You should then be able to double click on the data in the wan_username and _x_wan_password fields to edit as appropriate.
Could try migrating to a different hosting provider maybe. Here's a list I just found on google[0]. I believe meteor also has a cloud offering called galaxy. I haven't followed meteor in ages so I don't know the state of things these days, but they used to run meetups. If they're still doing those, maybe pop in and see if anyone can help.
IMHO your problem is not so much the hosting provider, it's the lack of expertise. If you can, try to reach out to those who had expertise and see if they can help you. Your sites don't need to be up for that, you could setup a discord room or whatever as a war room to coordinate efforts.
If you had some sort of mailing list for your community, you could also try to reach out and ask for donations (once you've secured some expertise). You can setup an opencollective to try to get recurring income to fund hosting.
You're also going to need to generate some sense of urgency with all involved parties. Waiting a week and a half for a response is just asking for things to fizzle out.
[0] https://studio3t.com/knowledge-base/articles/cheap-free-mongodb-hosting/#summary
Try googling your IP and whitelisting that instead of taking the MongoDB Atlas auto-suggestion, sometimes they aren't the same: https://studio3t.com/knowledge-base/articles/mongodb-atlas-login-ip-whitelisting/
it's in mongo itself. It uses jsonschema as well (via the "$jsonSchema" type):
https://docs.mongodb.com/manual/core/schema-validation/
https://www.percona.com/blog/2018/08/16/mongodb-how-to-use-json-schema-validator/
https://studio3t.com/knowledge-base/articles/json-schema-mongodb/
We had this issue at my work and we basically applied logic about using the right tool for the job, and bribed them with this MongoDB IDE/GUI that we use called Studio3T: https://studio3t.com/
Generally the Pro Mongo devs just point out which use cases are a better fit for Mongo and we get them a license for Studio3T, which has a SQL Migration function which enables entire SQL databases or multiple SQL tables to be exported into a single MongoDB collection and vice versa. It also has a SQL query function which services as a translator between the two, so they don't actually need to be well versed on Mongo in the beginning. They haven't really complained when the tool is doing a lot of the leg work for them