Typically you'd use a TURN server to relay traffic which has on-going bandwidth costs.
What's a TURN server and how does it relate to enabling mobile connections with NAT restrictions to connect properly using the Multiplayer plugin?
Even then, you can't get to a perfect 100% connectivity rate. The Internet is simply too chaotic and unpredictable - sometimes connections simply can't be made.
Yeah sure, but Construct is marketed as a tool that enables cross-platform game creation where mobile gaming is a significant component, and since the Multiplayer plugin is a staple of the toolkit, I can't be expected to tell the users of my Construct-created games to only buy my games under the caveat that they're on a mobile connection that doesn't have whatever convoluted technical restrictions that are currently preventing me from getting the Multiplayer plugin to work.