You’ll need to delve into the mysteries of “load balancing” to accomplish this. The MWAN3 module from OpenWRT sets the stage for this functionality. Unfortunately, while it’s quite capable of doing what you want, it’s capable of a raftload of rather sophisticated things like splitting IP flows between a WiFi as wan and other WAN interfaces- as such…it’s a bit difficult to initially set up. You’ll want to read up on the following:
https://wiki.openwrt.org/doc/howto/mwan3