The Rendezvous server is only involved in connection initiation. Once the Companion and your browser are in contact, it has no continuing role. So if you get disconnected once connected, the problem is not the Rendezvous server.
What is the name of the ISP that is blocking the Rendezvous server? I may be able to contact them to find out why they put the block in place. I suspect that they saw some number of their customers connecting to it (meaning they are using App Inventor) and assumed it is a malware command and control server!
In light of that, and because the majority of my Companion disconnections happen to my phone, and rarely to emulated Companions, I am leaning towards blaming my phone' s battery saver settings for Companion disconnections.