The official instructions do not work:
https://www.mongodb.com/docs/manual/tutorial/install-mongodb-on-ubuntu/

$ sudo apt-get install -y mongodb-org
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 mongodb-org-mongos : Depends: libssl1.1 (>= 1.1.1) but 1.1.0g-2ubuntu4 is to be installed
 mongodb-org-server : Depends: libssl1.1 (>= 1.1.1) but 1.1.0g-2ubuntu4 is to be installed
E: Unable to correct problems, you have held broken packages.

It seems the insecure version of libssl has been blocked for installation by Canonical.