My bad.
I entered into the wrong container.
Using the command: “docker ps -a”, I see a list and status of all containers.
CONTAINER ID IMAGE
675b342808c0 rocketchat/hubot-rocketchat:latest
ad7b1cafb36c rocketchat/rocket.chat:latest
d1f4a1fa1c71 mongo:4.0
Instead of getting the bash shell on the container d1f4a1fa1c71 and doing a mongorestore, I was accessing ad7b1cafb36c.
After that all that was needed to do was bringing the backup.tar file to this new location, untaring it and doing a mongorestore.
That did it.
Although after logging in, everything (messages, channels) was restored except for all the configurations previously done on RocketChat on the old server.