Teamviewer will not connect to the internet from a different wifi network

I brought my parents laptop to my house so that I could set up TV on it so that I can connect to it remotely given they are both in their 80s and in lockdown for covid-19 and rely on me to 'fix' stuff on it.

I created a connection to it from my own laptop at home whilst on the same network and it all worked fine. I gave it back to my parents but when I tried connecting back at my house TV said my parents latop was offline.

I can't enter their house at present becasue they are isolating but over a very tricky call with my mum I could establish that their internet connection was active and working but TV on their laptop would not connect to the internet.

I have managed to collect it from them again and brought it to my house and TV on their machine immediately connected to the internet and I can connect to their machine fom mine again.

So... why would TV on my parents laptop happily connect to the internet over my wifi/broadband but the same laptop not connect to the internet on different wifi/broadband?

Amy and all ideas/help appreciated