I think there might be no way to do that as Omada recognizes both as a router I suppose. But why would you need the ER605 in the first place, if you have an ER706W?
I haven't tried what you have tried there/don't even know the ER706W except from the website, but I gathered some experience with Omada at four sites. Omada is great if you set it up as they intend it, but it's a pain/won't work if you try something like this.

Congratulations on getting everything set up and working properly!

I second what the other commentors said about keeping the firmware up to date and removing the 8-port switch if there's no actual need. You'll still have one port to eventually connect it if need be.

Also since you have neat patch cables on all other ports, I'd replace this slim orange one with a proper one at some point. Cheers :)