Please whitelist the following IP addresses:
52.7.54.48
3.83.70.104
3.66.2.188
138.197.110.173
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 52.7.54.48 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 138.197.110.173 to any port 22
sudo ufw allow from 45.55.63.67 to any port 22