Reverse Proxy to DigitalOcean Droplets works but not with External Sites hosted outside DigitalOcean

I have a droplet called MultiSite where I have an apache reverse proxy. I want to use this droplet for the incoming traffic of all sites that I support and I will be supoorting. I will be moving some sites to different droplets in the same account in DigitalOcean and other sites would be hosted outside DigitalOcean where my customers have their hosting and they are not willing to move it to Digital Ocean yet (I focus on small customers to provide my support so some they also hire from me the hosting and I will install it in DigitalOcean and other will want to maintain their local webserver or hosted webserver…) . I have done basic testing: 1.- Reverse proxy works with sites hosted on the same reverse proxy droplet in DigitalOcean - DROPLET (Multisite) reverse --> SITE 1 in DROPLET (Multisite) STATUS: WORKS 2.- Reverse proxy works with sites hosted on a different droplet in DigitalOcean (I tried with wordpress-prueba droplet). DROPLET (Multisite) reverse --> SITE 3 in DROPLET (Wordpress-prueba) STATUS: WORKS 3.- Reverse proxy DOES NOT WORK with sites hosted outside Digital Ocean. DROPLET (Multisite) reverse --> SITE 4 hosted externally in internet (http://88.XX.XX.XX) STATUS: DOES NOT WORK

Why reverse proxy for sites hosted outside DigitalOcean is not working?.

Thanks