This app was mentioned in 2 comments, with an average of 1.00 upvote
Thanks for that, totally forgot that option. But it doesn't change anything for me when the file is already downloaded to the phone. It's somehow annoying when you just want to switch off some lights and you have to wait to get to the button. For me when I open the app it takes 3-4 seconds until I see the connecting screen, looks for a half of a sec if there's a new layout and then the loading interface screen with the dots for another 3-4 seconds.
I'm coming from a self made raspberrypi home automation project with a HTTP API where I used an app which was just doing HTTP get requests associated to buttons, that app opened instantly. For those who are interested: https://play.google.com/store/apps/details?id=com.stochastictechnologies.tungsten
I wrote one for Android:
https://play.google.com/store/apps/details?id=com.stochastictechnologies.tungsten
There's also Tasker. Can't help you with iOS, unfortunately.