Right now, for WAN fail-over, users are recommended to install mwan3, which is unnecessarily complicated for a pure fail-over case because it also supports load balancing. But even then, a simple feature request to bring up the LTE connection only when the main WAN fails (because of per-day payment) was rejected as something that would be better implemented in a custom script.
Well, this is an attempt to reconsider this decision, as this is a core feature of some ISP routers, and therefore should not require scripting if OpenWrt wants to stay competitive.
I would even say that this should be a core feature, not requiring mwan3.
The logic should be as follows: the user must be able to designate connection A as an on-demand backup for connection B, and then connection A should be brought up only if connection B is down for some time, or up on paper but does not actually work (so a "periodic connectivity check" feature is a pre-requisite).
Note: my current LTE ISP does not charge per day of usage, but the old one (back in Russia) did.