Failover Routing - How to detect dead link?

I have a network that is connected to the internet through Router 1 through cable, and separately through Router 2 on LTE. It is desired that in a situation where both cable and LTE are available, Router 1 and Router 2 both route their respective traffic through Router 1's cable connection. Only cable (or Router 1) fails, Router 2's LTE link is used. Once Router 1's link becomes available again, both go back to cable. I am trying to set up a routing daemon (Quagga) on both Router 1 and Router 2, so the two devices exchange available routes through the OSPF protocol. However, one of the fundamental assumptions of this concept seems to be that the router's kernel notifies Quagga when its link is dead; as such, Quagga will treat a removed LAN cable or a completely non-responsive router as a "dead" route, but not other modes of failure, such as the LTE modem losing association with the cellular network or the cable being removed from the cable modem (both of which do not cause any changes on the "local" side of the modem). Thus, my question: How can I instruct Quagga, or any other routing daemon, to periodically verify a route (e.g. by pinging a host that should be reachable through that route) and treat the route as failed if those pings fail. Thank you for any suggestions.