I have had this working internally/externally for some time now, with all services on one W2008 R2 box,using a 3rd party SSL certificate. Today I tried adding a separate site bound to a dedicated addtional IP on this box for a completely different site to redirect.
This somehow broke RemoteApp for external users. on LAN or VPN users can still access RemoteApp, but all others still get the message "This computer can't connect to the remote computer because the Terminal Services Gateway server is temporarily unavailable. Try reconnecting or contact..."
I have since removed the new site and IP from this box.. it looked like my redirect also hit the "default web site" (which is where RDWeb is installed by default). So I removed the redirect and all the Applications (RDWeb, RPC,RPCwithCert) still seem to look ok.
I have everything setup to connect to the external FQDN which is the same as the SSL cert name. etc. I have checked all known online solutions for this and nothing seems to really apply to this specific issue. This server is now in production so any help would be GREATLY appreciated. Thanks in advance.