I already suggested that the aux switch is not loadbearing in post six above. It should be to code anywhere, the aux is just acting as a remote for the Master.
(one exception to the code issue is the attic light thing… Some codes in the US, but not all, actually require a loadbearing switch control as you enter an attic. Others will allow that to be a remote.)
Anyway, there are several brands where the network switches act work as remotes, And don’t have to be on the same circuit as the master. (Lutron is a good brand, but as others have said, not directly integratable with smartthings.) The Cooper I already mentioned works as a remote. Leviton also has one.
In fact, the GE’s, which are cheaper, are one of the only brands that require the aux to be on the same neutral as the master. That’s probably why they didn’t work in the OP’s set up, although as Ray said, you really need to diagram the whole set up to see how the current is flowing before you could know which switches will work where if you want them all to be loadbearing.
But once you decide only the master needs to be loadbearing, you have a lot of options. Then it’s really no different then turning the switch on and off from your phone. Your phone is just another aux in that context. The other auxes can be anything that’s powered by either the mains or a battery. Because they will talk to the hub, and It will talk to the master.
Again, the GE’s are actually the trickiest to work into this kind of set up because of that neutral wire requirement. Plus a physical traveler wire requirement. They basically add a radio to a typical nonnetworked switch set up. Many of the other brands start instead with the radio and think about how they want to make the light go on and off given that they have a radio.