Ubuntu apt-get problem “KEYEXPIRED: The following signatures were invalid”

apt-get update Problem

Ign:11 https://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.6 InRelease
Hit:12 https://repos.sonar.digitalocean.com/apt main InRelease
Hit:13 https://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.6 Release
Err:17 https://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.6 Release.gpg
The following signatures were invalid: KEYEXPIRED 1544811256
Fetched 1930 kB in 3s (601 kB/s)
Reading package lists… Done

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.6 Release: The following signatures were invalid: KEYEXPIRED 1544811256

W: Failed to fetch https://repo.mongodb.org/apt/ubuntu/dists/xenial/mongodb-org/3.6/Release.gpg The following signatures were invalid: KEYEXPIRED 1544811256
W: Some index files failed to download. They have been ignored, or old ones used instead.

Step 1: Find the Expired key
sudo apt-key list | grep -A 1 expired

Step 2: Renew the Expired key
sudo apt-key adv –keyserver hkp://keyserver.ubuntu.com:80 –recv-keys
, the one you copied from above

Step 3:
sudo apt-get update

Allow ufw firewall in ubuntu

enable firewall - sudo ufw enable
reload firewall - sudo ufw reload
disable firewall - sudo ufw disable
allow port ya service - sudo ufw allow port/service-name 
(like - sudo ufw allow ssh 
or 
sudo ufw allow 22
or 
sudo ufw allow from sourceip to any port 22) 
allow rule delete - sudo ufw delete allow port/service-name 
(like - sudo ufw delete allow ssh 
or 
sudo ufw delete allow 22
or
sudo ufw delete allow from sourceip to any port 22)