I realize it would take some R&D to create this, but you might be missing out on business from places like Airbnb, or families who’s children don’t have smartphones (or have old hand-me-down phones with old software, etc).
If a stand-alone keypad and firmware were available, with a simple 2- or 4-wire interface, it could be programmed separately with codes and “trigger” on/off status when a current & correct code is entered.
In the case of Airbnb et al., codes could be generated at random and renewed with each new tenant.
Such a device wouldn’t be needed by everyone. Do you think it’s worth the effort to design such a module?