A router with redundant SIM.
SIM A is primary and cheap, but capped (e.g. US Verizon’s 15/22 GB per month)
SIM B is expensive but provides fallback when SIM A overflows
Identify SIM A as the preferred SIM.
Set the router to shut down SIM A when the usage reaches the cap, rolling the connection to SIM B.
Consequently the expensive SIM B is used only when SIM A caps out.
But the router does not switch back to SIM A when a new cycle rolls 'round.
Add a third option w.r.t. for the “Failback to Preferred SIM when…”
Force the radio to switch back to SIM A after resetting the usage meter at the end of the billing cycle.
This now requires manual intervention since the only options to return to the preferred SIM are (1) SIM B idles for N minutes or (2) SIM B is connected for M minutes. Neither speak to this use case.
Strong wish, not urgent.