2019 model. Sony's help says that I should be able to find this option:
https://www.sony.co.nz/electronics/support/articles/00176514
There is no such setting.
The TV remote button was previously set to switch to HDMI-1 which an android TV box is connected to, so at some point it has been customized to that input. This has somehow been reset and now the TV button switches to the built-in TV channel app.
I've done everything except a factory reset of the TV as I can't see why such a reset would restore a setting item that should be there by default, and because it's a pain in the ass.
TV software is up to date.
Any suggestions?
EDIT: thought I had it with this guide to set up an externa