Using external MQTT server together with internal one


I’m building a watering system for my grandparents garden. Hardware and software is set up and now I have to program the automation.
I’m thinking about 2 scenarios to control the watering system.
No. 1 : server at my grandparents which is accesible from the internet. If I have to change something I have to log into their system. However, I would have to set up the system and buy a server (raspi or similar) which has to run all the time. Not my prefered solution.

No. 2: I can control the system remotely from my own installation. Easiest way I can think of is to use an online mqtt server like hivemq. However, how can I control hivemq from my and still use my own mqtt server which controls my apartments devices? Any ideas? I would really prefer this solution.

Kind regards

You would need to create a bridge. I haven’t had one setup for a while so can’t give your any example but you can have a look at this post which I used to set my bridge up