My phone stopped sending texts after a couple of days. What went wrong?

Nearly always the phone's own battery manager. Android's standard power saving is handled, but Samsung, Xiaomi, Oppo, Huawei and others each add their own on top and those will close a background app whatever the app does. Find your brand in the guides on the SMS page and turn its optimisation off for the app. This is the single most common reason a paired phone goes quiet.