Ubiquiti UniFi UAP-AC-LR

Is this what you are using or this:

Ubiquiti UniFi Long-range Ethernet Repeater (UACC-LRE)

Yes, that's the series.  My model my be slightly different, but it's that series.  I use it not as a repeater, but to broadcast a different SSID.  

Should the Unifi LR Range extender be listed in the VM Hub menu >>> Connected devices ?

Does the Unifi LR Range extender have an admin menu & can that still be reached ?

Good questions:  I can't see the UnifiAP-AC-LR in Connected Devices under "Ethernet" connections for some reason, but yes I can see it connected - and with an allocated IP address -  in other views.

I have Unifi network controller software on iPhone and on my macbook, but it doesn't seem like I can connect to - or even see - the device for some reason. I've also tried downloading Hostifi but I can't seem to discover the device.

This is what the Unifi manual says:   Steady Blue with occasional flashing:  Indicates the device is in an isolated state (all WLANs are brought down until an uplink is found).

Any ideas?

In order to use the VM Pods, the Hub will be set back to "factory settings" by the app.  It just will not work if any of the settings are changed.  So when you only had the Unifi, did you change any of the Hub settings?  Just a thought.

My VM Hub will not list as a Connected device any IPs below the DHCP range,
e.g. a static or reserved IP in the 192.168.0.1 ... 9 range.  But these IP work fine.

Using a web browser does the UnifiAP-AC-L have an admin menu at its IP,
if it does what does the rest of the network configuration look like ? 

With the VM hub in Router mode we could expect ...

IP in the 192.168.0.xxx range
Gateway to be 192.168.0.1
Mask to be 255.255.255.0
DNS if used, to be a valid DNS e.g. 194.168.4.100 / 194.168.8.100 or a Public DNS,
could be the same as the IP of the Gateway on a Hub 5.

To the 2 questions above:

1. No, I don't recall that I changed any settings in the Hub to enable the Unifi device.  It was set up some years ago, but I'm sure I've pressed the reset button a few times since then.  However, I tried turning off the pods and re-booting (not resetting) the Hub without the pods:  the Unifi device wifi started up (so I thought "ah ha"), but then it disappeared (so I thought "huh")

2.  Seems like the Unifi device has dropped off the Hub list of devices completely now though, so I can't find the IP address to test whether there's an admin menu presented.

Does isolated state mean network cable issue ?

Bear with me on this one, if you set the Unifi to have the same SSID as the Pods, does this work?  I'm wondering if the two SSID's are confusing the Hub for some reason?  

If you remain convinced the extra VM Pod has adversely affected your system turn it off, 
in fact turn off all the VM Pods get them out of the equation.