Install unmanaged switch, split traffic multiple cpe modem

I mean like, if i'm not using my device. and instead uses devices that openwrt support, and also uses official openwrt firmware. what is the expected behavior?

let's forget split traffic for a bit.

If I were to use device supported by openwrt with firmware that is officially provided by openwrt. how would I use multiple cpe modem (connected using ethernet) at the same time?

for now its fine if it doesnt work exactly, i just need general direction. perhaps i might purchase new hardware. I plan on getting Nanopi R5S if my current device not good enough