How to allow 2 different nodered server which are connected to 2 different wifi router

Are there any solutions or ways to allow two Node-RED servers which are connected to two different WIFI router to communicate with one another? like for example, I have one for house A and another one for house B. How can they communicate?

How about a cloud MQTT broker?

Good idea. I will try it. Because previously i try to use ibm watson platform...quite confusing.

Thanks. Problem solved.haha