If your grandmother’s house is on other network, you will have to set up remote access for that Home Assistant.
Then you can access that home assistant by doing this:
Go to => Home Companion App => Settings => Home (or the name of your Home Assistant) => External URL => [Put your grandmother’s Home Assistant Remote Access URL].
It should work.
I haven’t tried this personally, but I faced some IP Conflict and resolved it using this technique, hence I know this should work.
Do lemme know if it works.
Thanks and Cheers!