So, simple answer is that you’ll need either a phone signal, or a wifi connection to use the app. I had a similar concern, but havent found it an issue so far.
If you get somewhere and don’t have enough signal to immobilise it via the app, you’ll still have the reassurance that it can’t be started by someone else without the fob also being present.
Alternatively you can install the app on someone else’s phone/iPad etc. I have it on wife’s as well, so that we have the option of accessing it on differing networks, or if she’s not with me.
Just for good measure though, I also have a stoplock pro and an extra hidden measure. So if I can’t immobilise via the app, I have extra fallback measures….and if anyone does manage to start it with all of those, they’ll be doing very well indeed!