You can also try logging into the router and going to Advanced > Advanced Setup > Wireless Settings and using the "Backhaul Password" option. This should cause the router to generate a new, random, password, after which you might have to sync **all** your satellites, but that might resolve the issue for you.
It also occurred to me you might want to check the "Enable Daisy-Chain Topology" option on the same page, as it could be that it's disabled and your original satellite is attempting to connect to one of the newer satellites, rather than directly to the router, due to range or placement issues, but that you have this ability disabled. It's possible that enabling daisy chaining could help you.
Finally, you could also try using the setup wizard, and trying to re-add the missing satellite that way. I believe at some point in the setup process, it will ask if you want to add a satellite, and it might either add them all for you at once, or allow you to keep adding individual ones until they're all added.
If none of those ideas from either this post or my previous one work, then you can either try a factory reset on the entire Orbi system, or using a wired backhaul to the lapsed satellite. If none of those work, then I'm out of ideas.
