That is a tricky set up indeed. I presume you already had a look at this?:
https://portforward.com/help/doublerouterportforwarding.htm
Yes, I did,
thank you for that suggestion.
Even before I stumbled across that reference, that is how I ended up attempting to make it work. I assigned static IPs for the router and for my computer. This allowed me to daisy-chain the port-forwarding from one LAN to the other LAN and to the (actual) WAN (the internet).
Basically this means that I port-forwarded my computer thru the router it is connected to (to the "WAN" IP of that router). Then I forwarded the same port number (5100) thru the second router (using its LAN address range and the static IP address of the aforementioned router (that my PC is connected to).
It's actually not that challenging; one has to just be mindful of which numbers belong to what device/LAN. As I mentioned in my 1st post, this appears to have reduced the number of unsuccessful connections as reported in
Network Options in-game. What's interesting is that the game still reports a
Port-Restricted setup. I'm not sure if that matters but I expect that it means that things aren't completely fault-less either.
Before I attempted this, my connection
failure numbers were at least 50% (or higher), so things are better.
That reference is pretty good but some of the language is a bit confusing. I expect that for some it's fine while for others (like me), it just doesn't quite make things clear. That's definitely one of the challenges associated with writing this type of assistive documentation. I give them credit for trying!