Does not work on most European mobile providers as you are behind their NAT and they don’t allow incoming connection requests to ips behind their SIM divice. An easy workaround is to install a raspberry pi and use VNC or Teamviewer to access that client and from their jump onto your opensprinkler. I use this as there is no other way that is straight forward to achieve this.
You could alternatively install a VPN server (your NAS or raspi at home could do that job) in a location that allows incoming connections. Then you have your remote location connect to that VPN server and through advanced routing you could make the remote client’s network accesible from you home network to which in turn you can connect remotely via VPN from anywhere.