Ashley I'm sorry to say, but the Multiplayer is good/reliable only for LAN connections, anything else fails because of NAT.
I've done the following test based on your documentation about Multiplayer. It lacks stuff life lag compensation, binary transmission, bandwidth reduction etc, but it works even on mobile data.
I've made the server in Node.js using WebSocket library and application in Construct 2. All the logic is inside the application, the server only stores the list of users (hosts, clients) and passes the websockets (messages) between them.
The window from the left is the host. The other windows are the clients that receive the x,y coordinates of the object.
Just after doing my own websocket multiplayer test I've realized how complex is Construct's Multiplayer plugin and how much work was put into it.
If you don't want all that work to go to waste (because at is now, the Multiplayer plugin can't be properly used ) my advice would be:
1) Host yourself an official TURN server needed for multiplayer (like the signalling server). If the costs are high, make it subscription based.
2) Teach the users how to make their own TURN server. I've found VPS hosts as low as 2~3$/month that could run a TURN server, or to run it on the personal PC.
And don't forget to tell the users upfront that the Multiplayer plugin costs extra because of the online implications.