Here’s a UFW-friendly version of the rules they had on their page:

ufw allow from 146.88.136.0/21 to any port 80,443
ufw allow from 94.31.33.128/27 to any port 80,443
ufw allow from 64.125.78.224/27 to any port 80,443
ufw allow from 94.31.33.192/27 to any port 80,443
ufw allow from 64.125.78.192/27 to any port 80,443
ufw allow from 198.232.124.0/22 to any port 80,443
ufw allow from 27.50.79.130/32 to any port 80,443
ufw allow from 50.31.251.34/32 to any port 80,443
ufw allow from 108.161.176.0/20 to any port 80,443
ufw allow from 64.125.76.96/27 to any port 80,443
ufw allow from 94.31.33.160/27 to any port 80,443
ufw allow from 64.125.102.96/27 to any port 80,443
ufw allow from 64.125.102.64/27 to any port 80,443
ufw allow from 216.12.211.59/32 to any port 80,443
ufw allow from 50.31.249.226/32 to any port 80,443
ufw allow from 119.81.131.131 to any port 80,443
ufw allow from 70.39.132.0/24 to any port 80,443
ufw allow from 64.125.102.32/27 to any port 80,443
ufw allow from 64.125.76.64/27 to any port 80,443
ufw allow from 64.125.78.96/27 to any port 80,443
ufw allow from 216.12.211.60/32 to any port 80,443
ufw allow from 27.50.77.226/32 to any port 80,443
ufw allow from 119.81.131.130 to any port 80,443
ufw allow from 146.88.128.0/21 to any port 80,443
ufw allow from 174.36.204.195 to any port 80,443
ufw allow from 174.36.204.196 to any port 80,443
ufw allow from 37.58.110.67 to any port 80,443
ufw allow from 37.58.110.68 to any port 80,443
ufw allow from 158.85.206.228 to any port 80,443
ufw allow from 158.85.206.231 to any port 80,443
ufw allow from 94.46.144.0/20 to any port 80,443
ufw allow from 94.31.56.160/27 to any port 80,443
ufw allow from 94.31.27.64/27 to any port 80,443
ufw allow from 177.54.148.0/24 to any port 80,443
ufw allow from 185.18.207.65/26 to any port 80,443

Youtube tutorial of how to whitelist IPs in Digital Ocean with their firewall: https://youtu.be/1ScRU_zi4oM