Please whitelist the following IP addresses:
34.227.212.80
3.83.70.104
65.108.217.1
3.66.2.188
45.55.63.67
You could trying the following commands on your server to whitelist SimpleBackups by adding it's IP addresses to the firewall.
sudo ufw allow from 34.227.212.80 to any port 22
sudo ufw allow from 65.108.217.1 to any port 22
sudo ufw allow from 3.83.70.104 to any port 22
sudo ufw allow from 3.66.2.188 to any port 22
sudo ufw allow from 45.55.63.67 to any port 22