@nasKoThat's correct; there's no TCP port (or UDP port) listening. Thus, when I (and a friend) attempt to connect, there are no errors generated by the server, nor logs captured, because it's deaf to all connectivity attempts. To analogize, it's akin to trying a thousand different ways of displaying and verbalizing how 1+1 = 2, but the person you're attempting to educate is 100% blind and deaf. So that person just keeps humming along doing their own thing, completely unaware of your presence.
@EnigmaGreyTHANK YOU. This worked. I hadn't realized how much effort had gone into this (since I didn't seen anything online about this feature and my initial test of it failed [as you mentioned, UPnP enablement required], so I assumed [wrongly] that it was an incomplete feature while alpha development continues). I am now able to host and friends were able to join. Much obliged! Btw, having now gone through this process, I thought I'd provide two suggestions to improve experience for server hosts.
1. Server setup options: I noticed most the options had mouse-over tips. As a feature suggestion for server hosts, a mouse-over for the electricity/water shut-off would be helpful, too. For instance, the default for the easiest and second easiest modes are "600" and "14", respectively. I "presume" this means in-game "days" (i.e. unless modified from default, 600 or 14 hours real-life gameplay). That's somewhat inferable, but what isn't inferred is if this is an absolute time or a range time (i.e. water shuts off in 600 days or within 600 days). Also, if it's the later (i.e. within X days), the code might also be extensible to provide a min and max range (i.e. shut-off water/electricity between X [4] and Y [10] in-game days, for those that want some randomness, but not necessarily no water/power within the first hour of gameplay [if terribly unlucky]).
2. When using the server hosting approach you recommended above, there's a tool tip provided for hosts, so as to successfully invite people. Mentioning the UPnP enablement for router and OS would prevent others from experiencing what I had. Granted, most probably have these on by default; I had assumed (wrongly), from past hosting experiences, it would leverage the port forwarding rules I had setup.