[federation] Public chan on server A can't be visible, nor accessed by invited user from server B

Description:

title pretty self-explanatory.

Steps to reproduce:

1. Camille, user on server A, creates a public chan
2. Once chan is created, goes into users and click on "add a user"
3. Martin, user on server B, shows in the list as Camille has already exchange in private with him.
4. A message is show in the chan saying Martin has been added. Everything seems to went well.
   option: 5. Camille sends a message in the chan and even mention Martin in it.

Expected behavior:

• Martin should receive a notification he got invited in the public chan
• Martin should see the the public chan on his sidebar
• In the event Camille mentionned him (option 5) he should receive a notification
• Martin is happy and can express himself with other peers

Actual behavior:

(same behavior with/without optional step 5)

• Martin doesn't receive any notification
• Martin doesn't see the chan in his sidebar
• Martin can't find the chan in the repertory as it lists only local public channels
• Martin can't join the public chan, he feels excluded and he's sad

Server Setup Information:

both servers have the same conf

• Version of Rocket.Chat Server: 3.8.1
• Operating System: Linux x64
• Deployment Method: docker/kubernetes
• Number of Running Instances: 1
• DB Replicaset Oplog: activé
• NodeJS Version: v12.18.4
• MongoDB Version: 3.6.12

Client Setup Information

• Desktop App or Browser Version: ff 83 on both sides
• Operating System: macOs 10.14.15

Reproduce the issue with two federated Rocket.Chat 3.8.1 servers using the stated Docker/Kubernetes, Node.js, and MongoDB setup. Trace the invitation, notification, sidebar visibility, directory listing, and join behavior for a public channel; done means an invited user on server B can discover, access, and receive mentions from that channel.