You kinda blew my mind with this, for all that people want and request this (myself included), I didn’t remember anyone actually saying that they tested the synergized device LAN ports.
Well, you got me out of bed to go do an initial test. It gives out a 169 ip address with a gateway and dns server of the synergized device.
Now all 4 of the connections on my synergized device are currently down (long story) but it did give me an interesting result, I didn’t get internet at all. The synergy host has 3 perfectly working connections that by outbound policy should have routed fine if this link passed things back to the synergy host.
So, since it works for you, I am going to say that you are going out your starlink connection, your ap mini is handing out 169 ip addresses from the B One in the barn and routing out to the internet from there. No idea how outbound policy would be being applied if at all?
As to your actual problem, your synergy controller in the house has one free VLAN as WAN that you can use as the synergy link (as long as you are over firmware 8.4.0). This would mean that you should be able to:
connect say, LAN 1 to the fiber connection going to the barn, configure the synergy connection to use vlan 10 on LAN port 1, set LAN port 1 settings as a trunk port carrying untagged LAN and VLAN 10.
add a VLAN capable switch in the barn between the fiber connection coming in and the B One. In the switch should be set to trunk port on the fiber coming in port expecting untagged LAN and VLAN 10, then configure port one (or whatever port you want) to be native VLAN 10 and connect that port to the B One in the barn however you have that connected now. All other ports on the switch can be native untagged LAN.
If you buy a switch that has POE, for this, you can also eliminate the current switch in the barn for simplicity if you want, all the remaining ports on the switch will be on your internal house LAN.
I can’t give specific instructions on the setup for this since it is very different on each switch (even within a brand) but it shouldn’t be too bad to get set up.
If you end up with a switch that is capable of inter VLAN routing, make sure it isn’t routing between VLAN 10 and the untagged LAN.
I will point out that I have never actually tested this, I can’t see a reason it wouldn’t work fine since it’s specifically mentioned that the synergy link can be a VLAN, unless something about the synergy link stops you from being able to route the untagged LAN on the same port or something. I highly doubt it will be a problem but if anyone has tested this, chime in. Worst case, a switch capable of VLANs on the house side too would be able to work around this completely transparent to both the B Ones and all ports and settings on them would remain how they are now.
Edit:
Also occurred to me in the morning, you can actually use a VLAN in the same way to connect the starlink WAN back to the house and eliminate the barn B One entirely. That assumes you only have the 1 internet connection in the barn, if you are using the 5G connection in the B One at the barn for better signal, have other connections in the barn or plan to add more, then your current layout is fine too.
You need to be careful with VLAN configs but I run almost all of my connections back to my main building in VLANs through wireless links and then connect them to my synergized devices in that one location in a mix of VLANs and physical connections in to the devices. Once it’s set up correctly, it works fine.