"Oh and every time we set the router to 'modem only' mode, so that we can use it with, like, a business router, not their POS, the settings revert after a few days."
That must be something special about the business grade one then, or it's borked. My home one has been in modem-only mode since it was last replaced/upgraded some years ago and has never reverted back to factory settings.
Biting the hand that feeds IT
