@Kamdon,
Unfortunately, Deco doesnt have any features for failover connections and Link Aggregation is usually specific to our higher-end Archer Routers. No matter what, to get the best experience you will most likely be looking at adding some sort of hardware to your network configuration.
If your connection is relatively stable and you only need a connection in dire circumstances for essential use (not gaming) you may simply look at the hotspot/MiFi devices offered by your cell phone provider to bridge the gap.
If you need something that will automatically transition your connection, you will need to look at other solutions such as a high-end business-class switch or another device to bridge the two WAN connections.