sure. It should follow some kind of active/passive HA cluster functionality. I guess this can be implemented in different ways.
One obvious could be like VRRP, with in addition to the 2 gateways Lan IP-addr, also a virtual Lan IP-addr that the VRRP master normally own, but will be taken by passive node, if master fails, Clients sends SMS commands to the virtual address.
While I do not know the SMSEagle functionality, it seems from the picture of theirs failover, that they use an alternative, by doing the keep-alive between the 4G ports and both having the same Lan ip-addr, but the backup unit having the Lan connection disabled until keep-alive to master fails.
Well, these 2 alternatives are actually different, in that the first is a Lan connectivity failover, where the second is more a Wan connectivity failover. Both could be useful, but I guess my main priority would be the first Lan connectivity alternative.