I have something similar on 1570 dual internet (WAN primary cell0 secondary). After a few firmware updates (requested by CP support), all the traffic goes over LTE, and on WAN only ping probes (successful). WAN is primary active, any routing over wan (GUI/clish) ends with the inactive state. Default gateway over WAN is not visible on gui/clish, only over LTE (cell0). WAN is not answering ping requests. The only traffic is the ping probe by system (answered) and the probe state is green for all servers.
But "route add target_ip gw gw_ip" from linux - works. Route appears in gui as a system route and appliance works perfect (until interface down/restart).
Any idea how to force WAN to work, without forcing it at system level? By the way, where is the best place to place ifup-WAN command?
I guess about /31 netmask on WAN, but it is now common ...