Can someone take a look and make sure I understand this right about reverse proxy

Ur understanding is right. 

Internet -> Ext IP (natted on firewall to virtual server IP F5) -> virtual IP F5 -> PoolMember (internal Server IP). You need to create two virtual servers. one with port 80 with the default irule for redirecting to 443. second virtual server with port 443 running with ssl cert within client-ssl profile. Maybe server-ssl profile if the internal server runs with https or withount server-ssl if it runs on http. 

maybe share some screenshots and we can try to find the error. external dns is working fine? ur landing on the right ip? there is so much to check.