Can not connect to mongodb on cloud server

[boh5]

I can connect to mongodb on my cloud server by mongodb compass and navicat, so I am sure that the username, password, host and port is correct, and the firewall configuration is correct.
But when I connect using flask-mongoengine, I get exception:

pymongo.errors.OperationFailure: Authentication failed.

I can connect to my local mongodb without password by flask-mongoengine.
My configration is like below:

MONGODB_HOST = 'xxx.xxx.xxx.xxx'
MONGODB_PORT = 27017
MONGODB_USERNAME = 'username'
MONGODB_PASSWORD = 'p@ssword'  # There is an `@` in my password, and I have tried use `%40` instead of `@` but not work
MONGODB_AUTHENTICATION_SOURCE = 'admin'
MONGODB_DB = 'my_db'

The versions:

mongodb server == 4.2.6
flask-mongoengine == 0.9.5
mongoengine == 0.19.1
pymongo == 3.10.1

Thanks!

Update:
I found that it seems a bug.
If I provid individual settings, flask-mongoengine do not support authentication_source setting, cause the Authentication failed.
If I provid dictionary settings with authentication_source: 'admin' like below, connect to mongodb success.

MONGODB_SETTINGS = {
    'db': 'my_db',
    'host': 'xxx.xxx.xxx.xxx',
    'port': 27017,
    'username': 'username',
    'password': 'p@ssword',
    'authentication_source': 'admin'
    }

[mromagnoli]

you saved me! It was happening to me when connecting to my mongo container

[insspb]

Please take a look at #429, I think changes from there will help.

[insspb]

Connection/Flask settings documentation updated in #477. Different settings options described. All options except list of dictionaries are deprecated since next version (2.0.0). Please refer to current documentation for more info and settings update.