There was still an issue whereby if the UPS is fully exhausted and my pfSense router loses power, it would not automatically reboot when mains power was restored. By the way my pfSense router is built on this fanless mini-PC:
Since that router is the heart of my network serving VLAN’s, DHCP and routing traffic between my internal networks and to the Internet (via the untrusted vendor-supplied ADSL router) if it is down, the network is down. In the worst-case if I’m away at the time then I need to get my local support engineer to hit the power button for me.
Fortunately - the fix was very simple. The BIOS version on my the mini PC that is my pfSense router has an option to boot when power is restored:
Now if power is fully exhausted and everything shuts off, when mains power is restored my pfSense router will boot along with my Unifi US-24-250W switch and the Unifi nanoHD AP which is powered by the switch via PoE+ - meaning my network infrastructure should all come back online smoothly.